On a warm spring day, I followed local graffiti photographer and urban explorer Michael (whom the Express agreed to not fully identify) across a green stretch of park lawn. We passed kids playing in the grass and a couple lounging on beach blankets, their heads almost touching as they talked. The weather was perfect for lazily soaking up the sun, but Michael's walk was straight and determined, and I had to jog every few steps to keep up with my six-foot-four guide.

Out here, we were exposed, and Michael wanted to avoid drawing attention as we approached our destination. My bright red rain boots were not helping the cause. Luckily, the kids and the couple didn't seem to notice my conspicuous getup. And as far as we could tell, we made it across the lawn without arousing suspicion.

When we reached our target, I was immediately thankful for the boots. Hiding beneath an archway of tree branches and hanging vines, the square mouth of a concrete cave yawned before us. But to pass through the gray gap and into the tunnel beyond, we first had to step off the dry grass and trudge through a moat of knee-high bog water. I managed to totter through the short stretch of muck and algae-covered logs without losing my balance, and I was immediately rewarded with a large painting of Swamp Donkey, a character painted by the local artist Swampy, which looks like the skull of a beast with exceptionally long, curved tusks and horns like giant conch shells. The words "Friendly Guard Dogs," "Low Hangin' Felony," and "All My Friends Are In Jail" filled the wall space around the tusked creature.

This spray-painted welcome was just the start of what we had come there for. The tunnel ahead promised four and a half miles of underground graffiti.

"Remind me again why I don't need to worry about drowning," I shouted as I followed Michael through the ten-foot-high drainage tunnel. "Because it's not raining," he answered. His short response seemed logical enough. We're in the middle of a drought after all, but I couldn't stop picturing the shallow river at our feet turning into a wall of water rushing toward us. People have drowned in similar flood control channels, and I wasn't eager to add my name to the list of deaths.

Michael, who is careful not to enter any tunnel or creek with more than six inches of running water, regularly takes these sorts of risks. In the last year, he has explored this tunnel four times, and he's wandered through fourteen other creeks and tunnels in the East Bay — all for the purpose of photographing underground art and posting the pictures on Instagram. Known to his 6,873 followers as Hellagraffdotcom, he's one of the most consistent and dedicated local graffiti photographers on the social media site. Other local graffiti photographers popular on Instagram include Streetview (6,630 followers), Mrxclownxface (6,576 followers), and Pixelina (6,031 followers). With nothing more than hashtags to promote themselves, these amateur photographers often have as many (and sometimes even more) followers than the graffiti writers whose work they document. They consistently garner between 300 and 400 likes on their posts, and pleas for "trades" and requests to go out exploring litter their comments.

Graffiti, urban exploration, and photography have long gone hand in hand. It began with the birth of graffiti culture in New York City in the 1970s, when writers first started painting in subway tunnels to avoid getting caught. Soon, photographers followed, tracking down these underground locations to document the work before it disappeared. A relationship that's part call-and-response, part hide-and-seek arose: Graffiti writers leave a mark that declares, "I was here," and the photographer follows to say, "I saw you."

Today, Instagram and other online photo-sharing sites have dramatically changed the scale and speed of this urban scavenger hunt. Photos that were once collected in albums and shared in person at writers' benches (places where writers would hang out and compare snapshots and sketches) can now be shared with millions of people around the world within seconds.

One of the results of this relationship has been a huge increase in the popularity of graffiti — as well as its cuter sibling, street art — in the last decade. Even though graffiti is considered vandalism in the eyes of the law, art collectors regularly shell out thousands of dollars to purchase pieces of this once explicitly anti-commercial work. Bansky's stencil, "Kissing Coppers," sold at auction for $575,000 in February after being removed from a pub wall in Brighton, UK. Last year, his stencil, "Slave Labour," which was originally spray-painted on the side of a discount shop in London, sold for $1.1 million. In the 2013 rankings of global financial research firm Wealth-X, Los Angeles-based street artist David Choe tied with Jeff Koons as the world's fourth richest artist, with an estimated net worth of $100 million. Shepard Fairey, famous for his "Obey" wheatpastes and Barack Obama's iconic 2008 "Hope" campaign poster, has reportedly cashed in on graffiti's cultural cachet to the tune of $15 million. You are more likely to find Fairey's ubiquitous silk-screen work framed in galleries — including the National Portrait Gallery in Washington, DC, where his Obama poster now hangs — than plastered on city walls.

On a local level, Oakland gallery LeQuiVive exhibits artists who straddle both the street art and fine art worlds. The gallery's roster of aerosol artists includes Cannon Dill, David Polka, Nina Wright, and Irot Che, all of whom have illegally painted in abandoned buildings and on urban walls. Loakal Gallery, which frequently exhibits muralists and street artists, such as Chris Granillo, Nite Owl, and Reggie Warlock, has sold pieces for upwards of $2,000. GATS (Graffiti Against the System), the Oakland artist known for painting masklike faces with ropey beards, sold original pieces for $4,000 at his recent show at the San Francisco gallery Hashimoto Contemporary. Swampy's hand-finished screen prints fetch $350 on the website Paper Monster, which sells original work and limited edition prints.

As more and more graffiti and street artists are inducted into the clean, well-lit art world, urban explorers such as Michael venture into train yards, vacant warehouses, and military bunkers to find and document graffiti that is, in some cases, literally underground and definitely free of any commercial ties. For them, no framed print could ever compare to the personal connection and excitement they feel when they track down a piece in the wild.

"In art galleries, it's more for mass consumption. People dull down their work," said Michael. "Then there's the private stuff — the stuff where they go into these places and spend hours doing pieces that are ten feet tall by forty feet wide and they're just insane. You're not going to get that in any gallery."

Along with the thrill of what he calls the "Goonies adventure" of walking on floors he's not quite sure are going to hold up, Michael, who spends his weekdays staring at a computer screen as a customer service manager for an e-commerce website, also finds a sense of calm. "There's something about the effort that we put into what we do — the cuts and scrapes and bruises and soggy feet and bad backs — to find something you can't see in a gallery, because it only exists in this tunnel," Michael explained. "It takes out the noise. You're not thinking about yourself, you're not thinking about work on Monday, you're not thinking about girlfriends or ex-girlfriends or family or the next episode of Sherlock or Game of Thrones, you're just in the moment and clear."

On an unusually hot day in November 2012, Matt Henry and his friend Greg Miller drove to the South Bay's wetlands, parked their car, and headed on foot toward Drawbridge, a ghost town whose abandoned speakeasies, gun clubs, and brothels are now closed to the public. Miller knew that there was graffiti inside one of the sinking buildings, and he wanted Henry to see it. To get to their final destination, they had to ignore several strongly worded "No Trespassing" signs and then walk a mile along an operational train track with marsh on either side. Aware that one of the 100- to 200-ton beasts could come barreling down at them at any minute, the pair kept looking in both directions as they walked.

"Trains have to sound their whistle when they go from solid ground to marshland, so you hear them coming," Henry explained as he recounted the story to me. "But you don't want to rely on that."

At the end of the walk, they reached a bridge. On the other side sat the dilapidated town. With no marshland to slide into if a train came, they ran. The bridge was short, but it was long enough to give Henry an exhilarating (and perhaps addictive) rush of adrenaline. Once safely in the town, they found the prize they were searching for. On the outside of a crumbling cabin, Bay Area graffiti artist Girafa had painted a large cartoonish giraffe's face with bulging eyes. The small shack's ceiling had collapsed long ago, and only a few planks fringed the edges. With no roof to stand on, Henry climbed onto the top of the wall's narrow edge, and Miller snapped a shot of his friend gazing down at the crazed-looking giraffe.

Although the pair found the paintings without getting hurt, the journey wasn't without real risk. Seventy-six people were killed while trespassing near train tracks in California last year. Henry gambled with his life — or at least the threat of a trespassing ticket — simply to be one of the few people to see the Girafa piece.

That day was Henry's first taste of graffiti hunting. "We walked out there, took pictures, uploaded them to Instagram, and then I guess it kind of took off," said Henry. Since that first adventure, he has made a weekly habit of ignoring "No Trespassing" signs, and his Instagram account, Bayareawanderer, has swelled to 3,273 followers. His hobby has led him to risk asbestos poisoning in decaying warehouses (Henry wears a respirator to protect his lungs); walk along the bolted seams of an empty fuel tank roof as it groaned and popped below his feet; and wander through a field of spent casings and down the empty streets of an urban warfare training facility just to find out what was there (luckily, no one was practicing that day).

Before Henry started exploring, the San Mateo native was plagued by horrible anxiety attacks. He felt like he had plateaued at the ripe old age of 26. He spent most nights playing Call of Duty on his computer from 11 p.m. to 2 a.m. He was acting out adventure online, but he wasn't experiencing any in the real world. "I needed to get out," he said. "I'd lived here so long, and I didn't even know what was around me. I had never really been to Oakland or Richmond. The whole East Bay was a mystery."

The San Francisco State University student began exploring with his friend Miller as a way to relieve his technology-induced isolation. On that first trip to Drawbridge, he happened upon the perfect graffiti artist to pull him out. Girafa painted his iconic yellow-and-black character all over the Bay Area, often tucked out of sight. He also purposefully left clues to where he hid his pieces.

"After I painted, I would leave my one-flow signature on things to let people know something bigger was close by, much like dropping bread crumbs," Girafa wrote in an email. "They would survive longer than the actual piece itself going unseen by what I call 'poachers.' I'm honestly impressed by how dedicated and determined Matt is in finding the remaining pieces I painted years ago."

Girafa stopped painting graffiti after his 2009 arrest (he was subsequently convicted for felony vandalism and misdemeanor vandalism, which resulted in a punishment of six months of house arrest, $38,000 in restitution, and five years of probation). "I enjoyed my adventures of going to places that hadn't been painted yet and likely won't be seen for some time," wrote Girafa. "Painting places like that reminds me of burying a time capsule — waiting to be discovered by someone else."

That neatly sums up the appeal for Henry as well. "If Girafa could go around and paint everywhere, who gives a shit? The whole thing is that you have to hunt for it," he said. "You have to find it before the city does. It could go away. If it does, you miss it."

Instagram serves as a sort of treasure map to this hidden art world. Explorers scroll through images searching for geographic clues — businesses in the background, mountains, freeways, and hashtagged city names. These clues, combined with the use of Google Maps, create the X that marks the spot.

"Prior to Instagram and Flickr, graffiti photographers had to work a lot harder to gain the trust of graffiti writers," GATS wrote in an email interview. "Most of them were directly involved with the community. Now there is an entire social group of just urban explorer/graffiti fans that exist parallel to, but dissociated from, the graffiti community. They shadow the scene, retracing the steps of graffiti writers, but without leaving a mark."

GATS said that it blows his mind how dedicated these photographers are. "I'll crawl through a drainage pipe a mile under the city, through spiderwebs and water, to paint a small spot that I'm 100 percent convinced no one else would ever be crazy enough or motivated enough to find," he wrote. "Then the next week, there are photos from five different people on Instagram who went through the same thing to document it."

Graffiti hunters are a cagey crew. They discuss particularly hard-to-find graffiti spots with the reverence others would reserve for the Sistine Chapel (there's even a spot referred to as the "GATS Church"). And they do not reveal locations easily. "It's the uniqueness and the secrecy — and knowing that you're now on the inside of that secret — that makes it exciting," said Henry. "You start to see things differently. You start to see boarded-up windows and think, 'I wonder if I could get in there? I wonder what's on the inside.'"

Before they're embraced by this guarded group, would-be explorers must first prove their worth by tracking down difficult places on their own. Instagram comes in handy here as well. Henry looks through people's feeds to see if they have photos of tricky places before he agrees to go out with them. This gatekeeper approach can seem a bit elitist and somewhat silly — like kids posting a "Keep Out" sign on their tree house. But urban explorers have good reasons for holding their maps close to their chests. When word gets out about a spot, a rush of people go to find it. The more visitors a spot attracts, the more likely it is that someone will report the trespassers. This leads to repaired fences, boarded-up windows, and heavier locks on doors. In graffiti parlance, "spots get blown up."

After promising not to reveal any locations, I managed to convince Henry and his occasional exploring partners Rachel Escoto (aka Pixelina) and Tanja Baker (aka Teetonka) to let me tag along one sunny Saturday. Henry and I ride in the car with Escoto. Baker, a lean German with an appropriately stoic vibe, drives separately in her pickup truck with two German shepherds in the back (a professional dog trainer by day, she often includes her dogs in photos she posts to Instagram). A petite 38-year-old with bright red lipstick and an equally vibrant personality, Escoto keeps the conversation flowing, relating stories about chatting with muralist Chor Boogie and dealing with the cops while on a urbex/graffiti outing with Australian street artist David "Meggs" Hooke. Although Escoto has become friends with some of the artists whose work she photographs, her words are charged with the excitement of meeting a celebrity.

The car talk feels giddy and gossipy as we wind along narrow residential streets lined with hillside mansions to arrive at our first destination, the so-called Lava House. Constructed in the 1970s, the multi-level home built with black lava rocks was never finished, and was razed to its foundation in 2003. According to local lore, a volcano god cursed the residence for the theft of his rocks, leading to the freak deaths of several construction workers.

Henry has read up on the history and claims, "the owner said he doesn't care if people wander around the property. And if he won't press charges, we can't get busted for trespassing." This seems like a somewhat shaky guarantee. But I'm reassured by the fact that this is not a top-secret spot — it's well known to county residents, and is easy to find online. And besides a small, obligatory "No Trespassing" sign, nothing blocks our entrance.

Even in the bright sunlight, the ruins radiate a haunted beauty. A round concrete table sits near the base of the stairs and boats sail by in the glistening water below. Beyond, the view of San Francisco and the bridges spanning the Bay is stunning. It seems like a perfect place for a picnic, but this trio is not out for a leisurely Saturday on the coast. They fan out like cops clearing a house, each moving from partially built room to room, level to level, collecting their shots as they go.

"I feel like a kid in a candy store," Escoto tells me as I trail after her. She stops to peek in the entrance to a small storage area. Behind rusty barrels and discarded planks of wood, Swampy has painted his character in black and white. Next to the mythical beast, the artist painted the words "safe in my cave." Escoto takes her time to line up her shot and then we move on in search of the next treat.

We walk below planks interrupted with stretches of sky where a roof should be and past concrete walls sprayed with rudimentary graffiti. After twenty minutes or so, we round a corner and see Henry perched on top of a ledge above us, taking in the million-dollar views. Unencumbered by a heavy camera and light on his feet, Henry bounces nimbly between the rocky outcroppings and cement ledges. In the time it's taken us to move through two levels, he has already climbed down to the shoreline and back up through the property's switch-backing paths.

Back in the car, Escoto and Henry compare how many Swampy paintings they have photographed and where. A mutual appreciation for the spots they'd each captured on Instagram led to their first joint outing three months earlier, but they josh like they've been friends for years, tossing around names of spots — "The Ruins," "Three Canals," "The Castle" — and other artists — CCTV, Pemex, Oracle — like comic book nerds might talk about particularly rare editions in mint condition.

At the next location on the day's list, we park at the base of a dead-end street and climb through a grove of trees, our feet sinking in a thick layer of leaves. Tucked behind the trees and homes, we find two silos. While the Lava House begs to be explored, this place is so hidden and unexpected, I can't imagine discovering it without an aerial view. But others have clearly found their way here. Both silos are ringed with vibrant lettering near the ground and several ladder-toting artists have managed to leave their mark higher up — including Oracle, who has painted his big-eared character hanging from a noose with a ghost leaving its body. The words "Hang In There" are sprayed next to it.

In our email interview, GATS told me that he had never been inside an art gallery until he was an adult. "Galleries are great but I don't think they are accessible to most people," he wrote. "In a world where everything is mass-produced and made on a computer, graffiti is something that very much still exists in the real world." As I walk along the wide bend of the silo, checking out the diverse range of styles painted on its surface, I'm happy to be far from any sterile gallery walls. Putting a price tag on a piece of art creates a barrier, leaving those that can afford it on one side and everyone else on the other. Out here, the only barrier to entrance is effort.

Although he paints in public spots as well, Jurne has become closely associated with work in abandoned buildings and tunnels. He turned a former Jetro Cash & Carry warehouse into a legendary yard for graffiti writers that gained the apt nickname "Jurne's House," and his 2013 show, Diversion, at San Francisco's 1AM gallery featured topographical maps of the East Bay's network of creek tunnels, where he often paints. He and muralist Matthew Litwack also co-authored the forthcoming book, Beneath the Streets: The Hidden Relics of New York's Subway System. Through extensive interviews and photographs, the book delves into the history of graffiti in the city's subway tunnels.

"I really gravitate toward those secluded places, where you can be in the center of the city and escape it all, but you can hear through the street grates the sound of cars rolling by or someone walking their dog," he said. "There's a tunnel in East Oakland that I really remember, where I saw these workers' scribes in the concrete from the '30s and I thought, 'Damn, people probably have not been down here since then.' It's cool that in such a highly trafficked, congested city you can find these little nooks that feel like you're on the moon, like no one else has ever been there before."

In 2013, he saw that someone had kicked in the door to the Jetro Cash & Carry warehouse. Inside, the roof had collapsed in the middle, creating a sort of post-apocalyptic atrium surrounded by huge, smooth white walls. He decided to make them his canvas. He patched up holes from people trying to strip the building's wiring, and eventually put his own lock on the door. At 7 a.m., while most people were getting ready for work, Jurne would go to the warehouse to paint. He brought friends in and the white walls began to fill with candy-colored pieces that took hours, and sometimes days, to complete.

"Then, of course, it got discovered and it wasn't that way anymore," said Jurne.

The vacant building was demolished following a fire in January. Some hold up its destruction as evidence of the over-exposure — and ultimate destruction — of graffiti spots, in part because of Instagram. Although Jurne thinks it's inevitable that spots will get blown up, he does concede that some on Instagram lack proper graffiti etiquette. "They'll say, 'This new spot on so-and-so street is dope. You should check it out.' You shouldn't put that out there."

"Jurne's House was definitely a shame," GATS wrote. "I've never seen such a pristine yard. He spent years painting that building in secret. Once photos of it popped up on the Internet, the search was on.

"I'm a little frustrated that places that used to run for years now only last a week or less due to Instagram culture and posting locations," he continued. "Hidden space used to be very elite. A select few would find and work in an abandoned building and only share the location with a handful of other graffiti artists who had earned their respect."

Few, if any, writers in the Bay Area are as visible as Mike. His YouTube video, "A Mile in Mike's Shoes," includes news footage of his name on the old eastern span of the Bay Bridge, shots of him tagging walls during the day as pedestrians and cars pass by, and images of him standing on a narrow ledge several stories high, spray-painting the Hibernia Bank Building's central dome in San Francisco. He's rightfully earned a reputation for being everywhere — including secluded spots.

"When you find your own little piece of heaven, it's yours. It's cool to share the photo or the memory," said Mike. "But to tell someone where it is could ruin that spot for everyone else. It's like going to a waterfall that only you've ever been to. You go there privately to get your own little bit of therapy. And then the next time you go, there are a bunch of mom and pops in wifebeaters barbecuing. And there are no animals around anymore. There's trash on the ground. You just took away that one piece of escape from somebody, and you ruined it."

Back in the tunnel with Michael, everything was pitch-black, with only our flashlights illuminating the way. We had only been underground for a few minutes, but already my heart was beating fast and my breathing was shallow. In the concrete tunnel, light and sound were quickly swallowed up or redirected. Even with our flashlights, the darkness felt tangible, like some sort of inky subterranean ooze circling around us. And I was almost convinced that zombies were waiting in the shadows. I could barely make out what Michael was saying even when he was nearby, and when I spoke, my words seemed to float away in a series of muffled echoes.

Along with all this sensory disorientation, I was also hyper-aware of the fact that I could only run in two directions — either forward or back. I tried to shake off the thought that this would be a really dumb way to die, and I kept sloshing ahead.

But there was comfort to be found in the network of storm drains that run beneath Oakland. Bright spray-painted words covered the walls that curved around us, including such supportive messages as "Keep Going," "Hey, Are You Alright? Don't Worry It'll Be Okay," and "Sometimes The Wrong Decisions Lead To The Right Places." And, of course, there were beautiful, carefully painted names and characters. Among the miles of pieces was a long-toothed golden GATS with heavy eyelids; a Girafa that blends the artist's trademark character with Super Mario; a series portraying Musk, an adorable mummy with an oversized head, leaping into a puddle; and another painting of the mummy crawling next to the words, "We Creep, You Sleep."

After slogging through a long stretch of darkness, we entered a pool of light spilling in from the grate overhead. I could hear children laughing. Aware that the people above might hear us as well, Michael and I both stopped talking. In the hushed air, I felt like I was walking on hallowed ground. With or without the incentive to paint pretty pieces for international audiences on Instagram, I could see why writers would want to escape to a place like this, and why they'd want to protect it.

This tunnel wasn't one of Jurne's pristine discoveries or one of Mike's waterfalls. People outside of the graffiti community have found their way in, too, but they've left behind whimsical additions, including a candelabra lit with battery-powered lights, a toilet protected by a lacy shower curtain (and accompanied by an old copy of Vogue), and, best of all, two swings hung with long ropes from the grates. I pulled myself onto one of the wooden seats and sailed back and forth. After I jumped off, I couldn't resist skipping through the stream. My legs were wet, but I didn't care. And I had long forgotten about the zombies and the potential flood.

After two hours underground, we emerged from the tunnel and into a pink and purple twilight. I felt lighter than I had felt in a long time, and I could see why nearly everyone I spoke to referred to exploring these places as therapy. As an adult, the world can feel deceptively known and predictable. We have our set routines and responsibilities. There is something magical in finding a world of rainbow colors and fantastical creatures concealed below these well-worn routes. Like the best museum art, the tunnel walls shook my brain off its normal course and reconnected me to a sense of mystery and awe. And I left wondering what other treasures might be hidden just out of sight.