In New York City, when night falls, a number of doors and less obvious passageways open onto another city. One of these is the mouth of the Amtrak tunnel that runs under Manhattan's Riverside Park. In December 2011, after five months of living full-time in the mundane city, I need a vacation, a respite not so much from the beloved city herself but from what cities increasingly consist of: light, noise, human and automobile traffic, crowded streets and stores and subway cars, trash and blackened gum on the sidewalks, the appalling tons of flotsam that wash up around us. For nearly half a year the only vistas have been vistas of human habitation. And so one cold night I take it upon myself to walk for nearly 60 blocks through the underground waste of the Riverside Tunnel, known colloquially as the Freedom Tunnel after Chris "Freedom" Pape, a graffiti artist whose murals made it famous among a certain subset of the population for whom spending time in dark tunnels is not unusual, and is even considered fun. My companion this night, Andrew Lynch, is one of this number, young and blond like me, but taller and less muscular, lanky with an easy stride. By day he sells real estate on the Upper West Side. By night—not every night, and increasingly fewer nowadays, but some nights even now—he's an urban explorer.

I'd come across Lynch's website while researching urban exploration in New York City. I reached out to him over email, and that led to a sit-down in a Starbucks near Grand Central Station—an unwise choice on the day before Thanksgiving—which led to more emails and finally to meeting up at half past midnight in the McDonald's at West 125th Street, our starting point for the incursion into the city's substructure. By then Lynch has already consulted with a friend who knows the tunnel and has conducted advance reconnaissance on points of ingress and egress. The way he's mapped out for us takes us over a chainlink fence behind a stand of trees at the top of an incline and then along a rocky path beside a highway. At one point we're forced to duck down behind the trees as a police cruiser glides past on the road below. Once over the fence—there used to be a hole in the chainlink, but Amtrak employees replaced that part—we try to be nonchalant as the headlights of passing cars wash over us. We're clad nearly head to toe in black—black wool peacoats, black gloves. I'm wearing the scarred leather hiking boots I inherited from my father, which he wore across the western United States and which are older than I am. It's the coldest night of the season so far, already around 32 degrees and dropping. Up ahead looms the gaping tunnel mouth, a deeper dark against the dark of the night around us.

This kind of exploration is illegal, and urban explorers, some of whom openly maintain websites about their hobby, occupy a dicey legal gray area in their public statements about it. The best "urbex" handbook offers extensive advice about sneaking and hiding. Urban explorers are seen by others, and tend to see themselves, as adventurers, renegade urban historians who map forgotten places. Rebels who hack the city. Usually they surf undetected through the city's circulatory system, but there are stories of arrest, fines, even death. Twenty-seven-year-old recreational climber Robert Landeta died in 1998 of a fall from the Brooklyn Bridge. Steve Duncan, a living legend of the New York urbex scene, scaled the exterior of the Cathedral of St. John the Divine on the Upper West Side, only to be arrested by dozens of police officers when he came down.

But Lynch and I have not yet transgressed. There's still time to turn back.

We enter, and are swallowed up by another world. What little light filters down from the grates above shapes out the contours of the vast cavern and the train tracks on their bed of ballast rocks. In daytime, these grates act as spotlights—concentrating the sunbeams on certain patches of wall, which, consequently, become prized real estate among graffiti artists. Their work is illuminated as in a gallery. But now, at almost 1:00 a.m., the tunnel is nearly lightless and the quiet is absolute. We're in an interregnum, a timeless space removed from normal time.

Ten feet into the tunnel, the silence, the uncanniness of it, is what strikes me most. It slows my heartbeat, calms my excitable city-boy nerves. It affects Lynch too. "It's like the city is this creature," he says, "and outside everybody's abusing it or changing it, but on the inside is where the heart is, and you see that it's just so still, so peaceful."

The other thing I can't help but notice is the graffiti. Lots of graffiti. The walls are festooned with tags, all of them jostling for space and attention in a place few human eyes will ever see except in passing, from the seat of a railway car, and then only during the daytime. As we travel farther we begin to see patches of wall that have been painted over. In the summer of 2010, Amtrak employees began painting over the huge murals. These murals, along with a welter of tags, had been created by graffiti artists over a period of years, and none were more accomplished than those of the man calling himself Freedom, who got there first with the most, and it was his murals above all that Amtrak sought to destroy. One of his artworks, of which pictures can still be found online, was a massive reproduction of Goya's The Third of May painted in 1992. Now it's gone. This erasure was done in an effort to diminish the undesired popularity the tunnel had acquired in certain circles, not only among graffiti artists and the homeless—who once lived here in an extensive shantytown, and who still trickle in on bitterly cold nights for protection from the elements—but also among urban explorers.

URBAN EXPLORATION, ALSO CALLED urban spelunking and building hacking, involves finding, accessing and exploring parts of the urban environment that are generally not open for public use. It should be said, however, that urban exploration need not involve trespassing, and most enthusiasts are happy to go somewhere legally if the opportunity presents itself.

One explorer who managed to do most of his adventuring on the right side of the law is Carlin Getliffe. As a college student in Rochester, New York, he formed an underground art group with friends, the Tactical Art Underground. Their initial goal was to break into buildings, mostly inhabited buildings, and install art projects without permission. But they soon decided they needed an aboveboard presence, a way to fund their activities, so they started a university-sponsored urban exploring club. It became so popular that it outgrew TAU. Since the club received university funding, Getliffe and the others had to get permission from landlords before setting foot inside their buildings. But most were only too happy to accommodate them. Among the empty spaces they explored legally was the top floor of First Federal Plaza, a 21-story highrise in downtown Rochester. Its circular top used to house a revolving restaurant called Changing Scene, but it closed down in 1990. When Getliffe's group visited, all but the top floor of the highrise was occupied.

Getliffe defines urban spelunking for me as "the exploration of all the usually off-limits or unseen parts of human civilization." But to his mind it doesn't matter how you get access to these places. "Sometimes we could go places by getting permission that we never could have gone otherwise," he says.

The late Jeff Chapman, one of the hobby's greatest boosters, has described urban exploration as "a sort of interior tourism"—interior because rather than encouraging you to look outward and go elsewhere—like most tourism—urban exploration is about going inward, finding the elsewhere within and behind and above all that's familiar in your own city. Sites for urban exploration range from steam tunnels to storm drains to abandoned factories to bridges, and this list is by no means exhaustive.

Urban exploration has existed in some form for decades, but it really took off, Lynch tells me, because of the Internet. Early writings about the hobby reached tiny audiences. Among these were Eric Bagai's seminal 1990 essay "The First Hackers" and outdoorsman Alan S. North's The Urban Adventure Handbook, published in the same year. These gave way in the '90s to newsgroups such as alt.college.tunnels—which drew postings from future urban adventure luminaries like Matthew Landry—and uk.rec.subterranea. In 1996, Chapman published the first print issue of his zine Infiltration in Toronto. According to his book Access All Areas, it was in the editorial of this inaugural issue that he coined the term "urban exploration." Previously, the most common term for the hobby had been "urban adventure," first used by North and later brought to the Internet by Wes Modes, owner of the website Adventuring.

From there, the history of urban exploration diversifies and fragments further. A webring is created by Chapman that encompasses 18 websites across the United States, Canada, England and Australia. Books of urban photography are published. Clubs and secret societies of increasing sophistication are founded on a shared interest in urban exploration. One of these is Ars Subterranea in New York City, which counts among its members architects, historians and artists. Julia Solis, one of the group's founders, publishes in 2002 a German-language book about the New York underground.

The history of urban exploration intersects at key points the surface history of the noughties. In October 2002, some 40 or 50 militant Chechen rebels seized the Dubrovka Theater in Moscow during a performance and took more than 800 audience members hostage. It was Russian explorer Vadim Mikhailov, founder of a group called the Diggers of the Underground Planet, who led Russian special forces troops into the theater by a secret underground route, saving the lives of many hostages and ending the seige. The Diggers have made it their mission to explore the thousands of miles of tunnels beneath Moscow. Among their purported discoveries is an entrance to a secret subway system known as Metro-2, long-rumored to have been built by Joseph Stalin to transport important officials out of Moscow in the event of an attack. In the United States, the tragedy of 9/11 provoked heightened security in just the areas that urban explorers like to go. Penalties for trespassing were also increased, making it much more difficult to shrug off the risks.

For some of us, at least, the human drive to explore, even though most of the earth's surface area has already been mapped, even though it's no longer necessary in order to obtain food and shelter, endures. And in an urban environment where many public spaces are not open for public use, this drive becomes the urban explorer's urge to infiltrate.

Like other hobbies, urban exploration has its connoisseurs and its dilettantes. The dilettantes are kids who want to get drunk in an abandoned tunnel, or climb into an old factory so they can break stuff. The connoisseurs include Moses Gates, an urban planner in Manhattan who has made a pastime of climbing bridges under cover of night, bridges whose distinctive methods of construction he can identify and name: cantilever, truss span, suspension. He is the sort of guy who will climb a retractile bridge just to cross that type of bridge off his list. Now 36, Gates, talking about urban exploration, can sound a bit like Danny Glover in the Lethal Weapon movies, forever proclaiming he's "too old for this shit" while coming back for more. No longer, he says, is he willing to trespass just for the sake of trespassing.

One cold night in December, Gates meets me at a Starbucks near Wall Street to talk about urban exploration. It's only a few days before Christmas, and I'd been afraid he wouldn't want to meet so close to the holiday, but Gates, I learn, is Jewish, and he's aggressively indifferent to December 25.

Over coffee, we talk about the "rebel mentality" that tends to go hand-in-hand with exploring. "Of all the urban explorers I've met," he tells me, "I have the least amount of that rebel mentality—and you've seen how much of it I have." I haven't explored with Gates, but I know a few of the stories. Like the one in which, while visiting Paris in 2006 with his friend Steve Duncan, he and Duncan climbed the exterior of the Notre Dame Cathedral and rang the bells on its roof. They were arrested, but felt no remorse. Even the French police, according to Duncan, understood why they did it.

One difference between the two men is that Gates doesn't overintellectualize his hobby. "Steve will try to sell you on that, but I won't," he says. "He'll talk to you about the hydrological palimpsest or something." And he insists there's more to urban exploration than adrenaline-charged thrills. Nevertheless, an anti-establishment attitude remains intrinsic to the endeavor, and is summed up by Gates as "Fuck it, I'm going to go where I want."

AS WE WALK, LYNCH relates the history of the Freedom Tunnel. Although built in the 1930s by Robert Moses, use of it was discontinued soon afterward. During this long abeyance of trains, a shantytown grew up in the tunnel. Hundreds of people lived there, some with pirated electricity—an underground Walt Whitman might have sung of. (Whitman, in fact, was no stranger to tunnels. He earned a place in the history of urban exploration when, in 1861, he recounted for readers of the Brooklyn Standard his visit to the abandoned Atlantic Avenue Tunnel in Brooklyn.) The 1999 documentary film Dark Days captures the shantytown in its last minutes of life, before the tunnel was reopened for use by Amtrak and the denizens were evicted en masse.

I ask Lynch how he knows all this, trying to reconcile his adventurousness with a passion for history that seems downright nerdy. Later, I come to find that a passion for research and historical fact is common among urban explorers. Gates, who's writing an urban exploration memoir, readily owns up to his own "nerdy fascination" with the history of forgotten and abandoned spaces, and confesses that "lots of urban planners are into this stuff."

Lynch's interest in urban exploration began, he says, "with a respect for old buildings." He grew up in Troy, New York, a town of 50,000 on the east bank of the Hudson River, the child of historical preservationists. Troy's downtown was full of large historic buildings, but there was no economy to provide tenants, so they sat empty. "My parents worked for a very long time to save and preserve a lot of them," Lynch says. "And so, as a kid, I just spent my entire childhood going in old buildings as they were being renovated."

From this unorthodox upbringing he gained a natural comfort around construction sites and other hazardous areas. At the time, his father worked for the state, his office in a former factory on an island in the Hudson River. Half of the factory had been renovated and turned into loft-style office space; the other half was still closed down—his father told him you had to wear a "spacesuit" to go in there. To Lynch, it had the allure of the forbidden. His father's office was close to the family home, within biking distance, and around it was a park, so Lynch found himself there many days after school.

"I just spent years hanging out there and being accustomed to that," he says. Seeing him now, I can well imagine the towheaded little boy running around his father's office.

When he was 15, his family moved to Arlington, an unexciting suburban town six miles northwest of Boston. He was bored with his new surroundings but jazzed to be living near a city with a subway. Not long after moving, he found out from family friends that there had been plans in the '80s, never brought to fruition, to extend the subway out to Arlington. As a subway enthusiast and, later, an industrial design student at the Wentworth Institute of Technology in downtown Boston, this past that had almost been was fascinating to Lynch.

"One night, I was very stoned, and I was staring at a map of the subway system. And I started thinking, What would that have looked like? So I started doing some research, and I found all these maps, I found all these plans and I found all these old pictures. And it started getting me to see the city differently," Lynch says. "At this same time in Boston, they were doing something called the Big Dig. The Big Dig was taking a giant elevated highway that was built in the '50s and putting it in a tunnel. So all of downtown was just torn up, all a construction site."

Since he was used to that sort of thing, downtown Boston became his playground. He was still in love with the city then, and would take regular walks through its vivisected heart. By then he had dropped out of school and was trying to find his way. One year in the future, New York City waited for him. He wasn't yet an urban explorer, but he had already developed a photographer's eye for the city at street level. One day in 2004, out walking with friends and taking pictures, he came upon a giant pit in the ground.

"What's that?" his friend asked, pointing down.

"Big Dig," Lynch said.

"No, what's that?"

Lynch looked again. There were train tracks visible at the bottom of the hole.

It was too good to resist. They walked down the sloping side of the pit. It was clear that they were in the subway, but it took Lynch a minute to realize precisely where. The old maps came back to him. In the old days, when the Boston subway was built, there was a section of the city that housed a landfill. It would have been prohibitively expensive to run track through this area, so an aboveground section was built to run the train as an elevated out to the suburbs. In the '70s, the city knocked down the aboveground portion and buried the tracks, building new stations underground. Lynch was standing in an abandoned station where no trains had stopped in 30 years.

That was his burning bush moment, his revelation on the road to Damascus. The moment his eyes were opened. All around him were the accreted layers of history, waiting to be explored. The past was still present. This abandoned station proved it. And if he could find this place, he could find others like it.

Other urban explorers have similar stories. For many, the root of their hobby lies in childhood, though the catalyst may not have come until years later. Getliffe's father is an architect like Lynch's, but it wasn't until Getliffe moved to Rochester to begin college that he got into urban exploration. "When I first got to Rochester, I thought it was the end of the world. I absolutely hated it. It was dark and gray, and there were all these abandoned buildings everywhere," he says. "Urban exploration was a way to engage with it." Liz Clayton, a Canadian expat who maintains the website Infiltration.org, says one of the benefits of urban exploration is that it allows its enthusiasts to forge "a more intimate relationship to a place that can be alienating."

The Freedom Tunnel is an undeniably alienating space, not meant for human consumption. Which, of course, makes it just the sort of thing urban explorers dine out on. Just about every side passage—and my companion and I explore most of them—features a rusty metal gate secured with a rusty padlock, and graffiti is everywhere. The beauty of decay is a big deal among urban explorers, as evidenced by the subjects they choose to photograph and the titles of their galleries on image-sharing websites such as Flickr. Another thing I'm learning is that most urban explorers are also to some extent photographers. Some, like Clayton, are drawn into urban exploration through a prior interest in photography. The urge to explore, whether in urban environments or elsewhere, seems inextricably bound up with the desire to document, which in the time of Sir Edmund Hillary meant keeping a leatherbound journal in which you'd write your impressions of the landscape and notes on your progress and and which now, in the 21st century, means toting a digital SLR camera and maybe even a tripod for long exposures in low light, means at least an iPhone photo here and there for your Flickr account or website and maybe for posting to your social network of choice so that all your friends and followers can congratulate you and admire your daring and generally eat their hearts out.

By the time I first enter the tunnel, much that once made it a mecca for urban explorers is irretrievably gone. But the Amtrak workers seem to have given up with the job half-done, and there's still plenty of graffiti, although most of it, disappointingly, is more vandalism than art—pure ego, someone's attempt to declare ownership. But there's still plenty to document, and Lynch and I have come prepared. We're both toting Canon DSLRs, and, while exploring, much of our conversation pertains to matters of light and shadow, photogenic graffiti, and the like.

At last, after walking 20 or 30 blocks, we come upon one of Freedom's murals, a famous comic book-style masterpiece that tells the story of police evicting the mole people. It's all but painted over, as if someone was tasked to spoil it and did just that, enough to ruin it and no more. Just visible is a policeman in a yellow Dick Tracy trenchcoat struggling with an assailant, or a victim. A speech bubble over the policeman's head reads 'Drop the gun mole!' From the bottom of the panel, a hand is reaching up to clutch at the policeman's sleeve, and an accompanying speech bubble contains a cry of pain. Of the mural that once was, this fragment remains like the visage of Ozymandias come to grief in the desert.



THERE ARE CLIQUES WITHIN the urban exploration community, which is not really a community at all but a number of individuals with a common passion, a sort of distributed network that at various times has organized itself around handmade enthusiast zines and Web forums. To the extent that a community identity exists, it's far from monolithic. Jinx was an oddball group that operated in the early 2000s, led by L. B. Deyo and David Leibowitz, authors of a book on urban exploration in New York. Among the group's quirks was that no matter how unsavory the environment being explored, its members always dressed in evening attire, the men in suits and ties and the women in cocktail dresses.

The LTV Squad operates at the other end of the spectrum, aesthetically speaking. The LTV Squad website is like something out of the bad old Angelfire days of the Web, a skronked-out portal with ugly graphics and an attitude. Should you wish to join the squad, the FAQ informs you that its members "have no interest in your personal info. We don't want your address, credit card, etc. We just want to know who the hell you are so we can decide whether or not to kill you ... I mean, let you in. Of course." By way of membership rewards, the site offers: "All the serious explorers in NYC that matter are members, and anyone that isn't is a loser in life, so it's a good way to get acquainted with quality people."

The individualism of urban explorers makes it difficult to generalize about their motives. Gates, although he's wary of getting too abstract about his hobby, says that the "push for more openness versus more control" in the use of urban space is at the heart of why he explores. In his day job as a city planner, he advocates for the public's right to access public space. As an urban explorer, he acts on his beliefs. But even for him, the emotional pull seems to matter more. What I find is that many urban explorers seem to do what they do for deep-seated emotional reasons, but rationalize it after the fact, providing intellectual reasons to justify it.

"In very few cases, I think, is something intellectual the underlying drive behind it," Getliffe agrees. "The truth is, some people are incredibly attracted to it, and it's probably something on a very basic personality level that draws them to it."

Or maybe even deeper than personality. "I believe we are all born explorers," polar adventurer Erling Kagge tells me. "An explorer is not something you become. At three, four, five years old we are all explorers. But somehow—due to parents' expectations, school, et cetera—we stop being explorers."

In December 2010, one year before I plunged into the Freedom Tunnel, Kagge got there before me. Having already, at the age of 31, become the first person in history to complete the Three Poles Challenge—trekking on foot to the North Pole and the South Pole and climbing Mount Everest—Kagge, with no pressing territory left to conquer, read philosophy at Cambridge, wrote a few books and settled down in Oslo to found a publishing company. Two years ago, he was put in touch with Steve Duncan by a mutual acquaintance, and the idea for an expedition began to form. The concept of urban exploration fascinated Kagge. It was as if, having performed an almost unimaginable feat aboveground, all that was left for him to do was to go under the earth. He and Duncan spent three days underground, exploring sewers and subway tunnels, and their exploits were covered by The New York Times and NPR. Gates joined them for certain legs of the expedition.

"For me it's about curiosity," Kagge tells me by phone from London. "It's interesting to cross the city in a fashion that you haven't done before. I fall in love with the idea of doing something, whether it's skiing alone to the South Pole or exploring the New York underground."

When I press him about what it was like to be down there, he says it felt like "walking into the subconsciousness of the city." There is a negative beauty to sewer tunnels and other underground spaces, he tells me; they're "beautiful for everything that's not there," for everything they call to mind by its very absence. There's no fresh air, nothing made with aesthetics in mind, and yet the environment ends up being beautiful somehow.

What I'm calling the other city is in some ways the inverse of the surface city we all know. It's quiet where the surface is loud, dark instead of lighted, abandoned instead of occupied. Because of these qualities, it's a city you can establish an intimate connection with. Intimacy with monstrosity has always been the lure of Manhattan, and urban explorers have been the most successful in achieving it.

One can't possibly hope to know all the faces of New York. One can pretend, of course, which some lifelong residents do, and even some foreigners who travel to the city for a visit and find themselves unexpectedly staying on. Every New Yorker thinks the city is his city. But it is precisely these individual relationships between the city and the millions who inhabit it, or who in former ages passed through it as a way-station along the road of life, that make it impossible ever to hold simultaneously in one's mind all the aspects of the city so many take as their own. In the course of its history it has gathered to itself too many humiliations and failures, too many secret victories and betrayals, too much hope and despair ever to be fully known by any single individual. It is perhaps this sense of inadequacy beside the city itself, and all it has witnessed, filtering as it does through the trivialities of daily existence, that gives to the inhabitants their characteristic solipsism. And this self-absorption makes them incurious, blinds them not only to the intricate, multi-form nature of the surface but, more crucially, to the access points which lead to the secret, hidden New York, and which are scattered throughout the city, pearls before swine.

"You open the right door, and there's an entire other city that nobody knows about," Lynch tells me.

This becomes something of a byword between us—"the other city." The idea of a hidden New York informs the writing and mindset of urban explorers. A landmark of such writing, which, like many things inside the circle of urban exploration, is not even on the radar of most people outside it, while being glaringly prominent to those inside, is the book Access All Areas. Despite being authored seven years ago, it's still immensely informative, offering—true to its billing as "a user's guide to the art of urban exploration"—sections on everything from training to equipment to stealth tactics. The book is chock-full of actionable tips: "When you're dealing with a creaky floor, try to find the part of the floor that creaks the least—generally, floors and stairs are most secure near the walls and in the spots above floor joists (that is, where the rows of nails are)."

Chapman, the book's author, drew on his personal experience as an explorer in Toronto as well as the archives of his zine, Infiltration, to compile the book. It was published in July 2005. Chapman died of cancer less than four weeks later. Clayton, who had collaborated with Chapman on the writing, editing and production of Infiltration, wrote the book's foreword and now maintains Infiltration.org, where you can still purchase every issue of the zine, in Chapman's honor.

Some time back—she won't say how long—Clayton moved from Toronto to New York, where she currently lives. She acknowledges the reality of the "city as kaleidoscope," intimidatingly multi-form. This kaleidoscopic quality is what compels some people to keep to their own little corners. It's the reason so many Chinese immigrants in New York rarely set foot outside Chinatown or Flushing, why Williamsburg hipsters seem possible only in Williamsburg. Clayton deplores this insularity. "Being curious is to be encouraged," she says. Urban exploration creates a personalized encounter with the city. "The feeling of freedom comes from not having a tour guide."

"Urban exploration is an inherently emotional activity," Getliffe tells me. For that very reason, it's difficult to give up. Gates, Duncan and Clayton are all well into their thirties. But Clayton, though not as active in urban exploration as she once was, adamantly refuses to call herself retired. If there's a place she wants to explore, she says, she'll go. This chimes with something Gates told me. "Citizens of a city have a certain ownership of it, and they should be able to explore their city on its own terms, to go where they want to go," he says. A passage from Chapman's book also comes to mind: "This hobby isn't about the quest for danger so much as a willingness to accept certain levels of danger in the course of the quest to discover and document forgotten or neglected realms."

IN THE FREEDOM TUNNEL the city is restored to stillness and silence. Indeed, the great value of such a space consists of its silence, which has been stored up in the absence of human beings. For many, this is the great discovery of urban exploration—the healing quietness of abandoned places, of an old sugar refinery in Williamsburg or an abandoned power station in Yonkers. Among other draws, it's this meditative calm that keeps Gates returning to the Parisian catacombs that he loves and to other pockets of timelessness. "There's a quiet that you get that is very difficult to replicate anywhere else," Lynch says. "You're so in the moment. It's like being one with God. Honestly. I'm not trying to be blasphemous, that's as close as I can come. It's why I've tried to get into meditation, it's why I still do urban exploration, because for that brief moment I am so at peace."

Even when something disturbs the quiet, as Amtrak trains do in the Freedom Tunnel during busy hours, it seems part of the mood, merely something that arises from the silence and returns to it. And yet in the very midst of this silence without human interruptions are signs of life, of those who've gone before you: a graffiti signature, a huge mural, a chair perched upright in a tunnel where no chair should be. This is a further discovery—the continuity of exploration, the community of infiltrators. These signs are for urban explorers the equivalent of the flags on the summit of Everest, evidence that others have passed this way and that others will come after you.

A final discovery is made upon emerging from the tunnel or descending from the bridge into the ordinary night. You become enmeshed again in the lights and traffic of the mundane city, the antithesis of the secret world you've just visited (and explorers by definition are always visitors). Only when you feel the net of modern life on your skin do you realize that you'd forgotten for a brief space what it felt like to be caught in it, that it was this freedom from society that constituted the peace you so recently enjoyed. But now the net has you once more, and the lines snap tight. It's like getting away from the earth awhile and coming back, as Robert Frost wished to do. Just after exploring, the everyday commerce of humanity seems indescribably banal, and the world a noisy place populated entirely with louts and vagabonds. This is true of all exploration, but with most outdoor expeditions there is a gradual transition back to civilization, back to reality as it's commonly understood, a packing up of gear and a long car ride through rolling hills or terrain emphatically flat or a plane flight with the Alps diminishing in the distance.

With urban exploration there is no period of transition, no time for adjustment. Especially when exploring in the daytime. The disembodied voices of children playing in Riverside Park that filter eerily down to you in the tunnel below resolve aboveground into actual living, breathing Technicolor children. Because of the secret world's proximity to the mundane city, it seems especially astonishing that the surface inhabitants should be entirely ignorant of this universe next door.

GLENWOOD POWER STATION SITS on the east bank of the Hudson River, a long-abandoned plant in which the detritus of vanished industry is left to rust. From the elevated platform of the Glenwood stop on the Metro North line, you can see, near at hand, the power station's two massive smokestacks rising up against a china-blue sky. Away to the northwest on the farther shore of the Hudson are the Palisades. These cliffs, famous for their steep sides, range in height from 300 to more than 500 feet along their length, and lie opposed to Manhattan across the swift-moving waters. A stark reminder of nature within sight of the city's equally sheer skyscrapers.

The night before my incursion with Lynch into Glenwood Power Station, I can't sleep. I finally settle down at 7:00 a.m., planning to wake up at 11:20 to catch a train north. A short rest, to be sure ... but even so I keep waking up early to check what time it is on the phone beside my pillow, a phone which I've armed with four alarms spaced out at intervals of 15 minutes, each a different sound, because the last thing I'm going to do is let myself oversleep. I'm not the only one suffering from temporary insomnia; Lynch tells me he can never sleep the night before "something big." He'll be riding up from Grand Central, and, since I live in Riverdale in the Bronx, I'm going to catch his train at Spuyten Duyvil. After we pass the Riverdale station we're out of New York City proper. A vague but definite boundary has been crossed.

A few nights before the trip, Lynch and I grabbed a table in the David Rubenstein Atrium at Lincoln Center to lay our plans. I hoped to use the opportunity to ask some final questions about urban exploration. Ever since my experience in the tunnel, and in fact long before that, since the night when I trespassed in an abandoned Chinese theme park in Central Florida, an understanding of why I felt drawn to explore these places had been growing in me. I was now speaking from inside the circle. But I wanted to hear it from him.

"Most of the time, I don't talk about this with people," Lynch admitted, "so it's interesting for me to talk about it, because I connect it with other things in my life that I never thought about." Our conversation returned to the solace of uninhabited spaces. "That extreme focus and the quiet—words can't describe it," he said. "It's just a moment I remember. Every time I do it, it's like a drug that hits you—it's like, 'This is why I'm here.' You don't want it to end."

We disembark at Glenwood from separate cars and meet up on the platform. To our left, the power station rises up like a monument to a forgotten past, my own or someone else's, maybe a whole nation's. That day in January, it's five degrees with the wind chill, and my hands are already getting cold. Getting in is child's play: the plant is actually divided into two halves, between which is a sort of narrow courtyard that runs west the length of the buildings and is accessible via a rusted metal gate. One door of the gate stands open, and the chainlink fence someone's put in place to seal it off has been cut away. Trees and creeper vines skeletonized by winter fill the courtyard. I can't help but remark on the plant's resemblance, up close, and largely because of the panes of glass set into recessed arches, to a house of worship. "That's because this building dates from the period when industry was the new god, and people approached it with awe," Lynch tells me. Urban exploration itself, he says, "is arguably a religious experience. And especially because some of these places, the bigger ones—sometimes it's like walking into a cathedral."

Once we enter the plant itself, there's absolutely nothing interesting to see. Nothing, that is, except for boards and bricks, nails and bits of glass, ladders, dismembered furniture, Tropicana fruit juice bottles and potato-chip bags, deep wells of ice in huge pipes, barrels tipped over and giant spools that once held cable, shattered windows, dirt and leaves and icy mud that have blown in since the plant was closed, bird droppings from the flocks that must roost here in the summer, obsolete control panels and rusted-out engines of obscure purpose—the detritus of civilization, leavings of a whole way of life that ended not with a bang but a whimper. Empty spraypaint cans here and there and ubiquitous graffiti, whole walls covered with it, and some of the tags in places almost impossible to reach, so that the artists must have performed unthinkable feats in order to leave their marks. According to Lynch, they hang upside down from the rafters.

The structural integrity of staircases, balustrades, and grated-metal catwalks is questionable. Since we're exploring in the daytime, we haven't brought flashlights, but our smartphone flashlight apps serve just as well when we head down into a dark lower level. We're extra careful down there, watching out for rotting wood and exposed nails. One door gives us pause. On its surface is scrawled DEAD hermaphrodite and assorted other dead stuff, the letters smeared but legible in our flashlight beams.

At some point my throat begins hurting, and whether it's the crackling cold or airborne asbestos or other foreign particles kicked up by our boots in the sub-basement–lead dust and rust shavings and so forth—I'm not sure. Lynch says he wouldn't be surprised if it were any of the above. We're intruders, after all, and there are some things that are better left at rest. (It's worth noting that extremely conscientious explorers wear face masks with HEPA filters when exploring places like this.)

We go further and further into the building, until it seems much larger on the inside than it appeared from the outside. It's an astonishing place, but the feeling of meditative calm remains elusive. Lynch says he's too aware of himself, whereas the essence of the feeling is to be fully present in the moment while letting go of your self-awareness, achieving a sort of total focus and oneness with the environment you're exploring. I can't help but feel guilty that my presence might be preventing him from attaining it.

Which is not to say we aren't having a great time. While traversing a latticework of wooden boards, Lynch confides to me that he's feeling "like a kid in a candy store." I know what he means. Everywhere we turn, a new vista opens up, new objects of interest present themselves. Lynch has his DSLR with him, and sometimes he pans the room with the shutter going rapid-fire, clickclickclick, like a revolver cycling on empty, while at other times he's patient, an auteur, finding what he wants and framing the shot and pressing the shutter release just once before moving on. Despite the biting cold I keep taking off one or the other of my black leather gloves so that I can manipulate the touchscreen of my iPhone and take my own pictures, having no DSLR with me on this excursion as I did in the Freedom Tunnel.

I soon lose track of precisely where I am in relation both to the building itself and to the four cardinal points of the compass; the light filtering in from the broken roof is a sort of all-directioned light that leaves no shadows where it falls. Exploring the plant begins to remind me of a high-speed tour of the Metropolitan Museum of Art that I took once, room after room of wondrous things of such quantity and inexhaustible variety that it taxed the eye, only here instead of German silver chalices and Renaissance oils and illuminated Persian manuscripts are cabinet doors off their hinges, metal rods and tools, a giant mural of a masked face surmounted by an eagle's head. Some high-ceilinged rooms are like junk shops. We come upon an inexplicable pile of planed boards, such a huge quantity that I'm surprised someone hasn't carted them off for use elsewhere. And there's plenty of useable metal lying around. "I know a guy who uses scrap metal," Lynch says. "If I let him in here ... I'd want a 20 percent cut."

In a sort of loft-style room that's mostly empty we find a nest of dirty blankets, unmistakably someone's bed God knows how long ago. Some of the graffiti is date-stamped and very recent.

We struggle for metaphors, things to which we can plausibly compare this unpeopled dominion. I think of a post-apocalyptic world, Lynch of the environments in the classic computer action game Half-Life.

When we first approached the building, I'd wondered aloud whether we could get up on the roof. Eventually we find a ladder that leads up there, and find ourselves with the Hudson laid out at our feet. We have an unobstructed view north out over JFK Marina & Park with its pagoda and manicured expanse of brown grass and, further north, the Tappan Zee Bridge. We consider more than half seriously how fun it would be to have a picnic up here in the warmer months. Looking around, I realize that this is what continues to draw dilettantes to urban exploration: the promise of an off-the-map hangout, the idea of a secret place all their own, a place that's as much out of reach in their ordinary lives as a fata morgana of some distant paradise that lies beyond the curve of the earth.

Lynch wonders openly about the future of urban exploration. The Internet is proving to be a double-edged sword. In the early days, it helped a community to form around the shared hobby; email lists, newsgroups, websites and discussion forums allowed explorers to share information and find buddies for excursions. But now insider knowledge is spreading too widely; everyone and his mother seems to be getting in on the game. "The places that were once sacred are touristy now," he says. And yet it's hard to blame the arrivistes for following the same impulses he followed in Boston.

"I believe exploration will endure," Kagge says. "We are born curious. We are born wandering over the next hill. And we are born wanting more space around us. I think we will always be explorers."

For people like Lynch and Kagge and Duncan, the drive to explore has not only survived adulthood but is deeply ingrained in their identities. Yet most urban explorers struggle to express what lies behind this drive.

Asked why he keeps going back for more, Lynch makes a comparison to sports. "It's the same reason that race-car drivers do what they do," he says. "It's because, for that moment where they're sitting there, one with the car, and they're in that moment...." He trails off. "You know, everyone has that thing that lets them get into that headspace. Whatever it is. But that's what I find when I do it. It's Zen, as Zen as you can get."

His words put me in mind of a quote by Camus. "As a remedy to life in society, I would suggest the big city," Camus writes. "Nowadays, it is the only desert within our means."

No offense to Camus, but the surface city isn't nearly desert enough. My own remedy is the other city, the city you access as an urban explorer. In terms of silent solitude, Zen-like peace, discovery and self-discovery, it is the truest man-made desert you can find.