Once it’s clear the cruiser is gone, Haefner doubles the truck back to his original spot. We tumble out and sling on backpacks full of food, water, and gear. The camera equipment alone—digital SLRs, lenses, lights, and tripods—works out to 20 pounds per person. Attire is dark clothes in layers: thermals, sweaters, down jackets, hats, gloves. We silence our cell phones and don industrial-strength headlamps, though we won’t use them unless it’s absolutely necessary. As always, the men have scheduled their expedition for the night of a full moon, to reduce the need for artificial lighting that might attract attention. (Photographs taken under the full moon can appear as brightly lit as daytime shots, given a sufficiently long exposure.)

The two-and-a half-mile hike takes us down a pitted trail with gorgeous but perilously distracting views: deep ravines, improbably large sandstone outcrops, vistas of Los Angeles, and a vast dome of sky. Coyotes howl, sometimes in the distance but at other times disconcertingly near. On a few occasions, a bump or crack sends us stooping for cover, but these prove to be false alarms. Halfway to our destination, we jog along the shoulder of a quarter-mile of road bookended by two sharp turns. Here cruisers can appear without notice, and there’s no scrub to duck into—just dirt and barbed wire. Much of the land is permanently bare, a result of contamination. “If a car comes, just hit the deck,” Freskos advises.

A great horned owl heaves off a power line, startling everyone. But no cars show. Off the road again, the trail forks several times, and for a moment we’re lost. “We go through this every time!” Haefner whispers, exasperated. We wait while Haeber consults his phone. (It’s perhaps fitting that the military trespasser’s toolbox includes many items pioneered by the military, such as GPS and satellite imagery.) As soon as we crest the ridge ahead, we can see our first destination looming in the near distance: the Alfa test stands, which once flared as bright as the sun.

The scale of America’s Cold War expenditure dwarfed everything that came before it. In today’s dollars, the Revolutionary War cost the country $2.5 billion, the Civil War $84 billion, the First World War $350 billion, the Second $4.3 trillion. During the Cold War, America’s tab just for nuclear weapons ran to $5.5 trillion. Thanks to the Cold War, the US Department of Defense owns more building space—2.3 billion square feet—than any other entity in the world. (These holdings are roughly 40 years old, on average.)

Yet most of our Cold War matériel is nearly impossible to see. Civil War buffs can visit more than 200 forts and 100 battlefields; there are major national museums devoted to World Wars I and II in Kansas City and New Orleans, respectively, as well as the stunning Holocaust museum in Washington, DC. But the Cold War remains practically invisible. There are repurposed fallout shelters scattered across the country, small missile museums in California, Florida, Arizona, and South Dakota. A chunk of the Berlin Wall anchors a row of urinals in the men’s room of a low-rent Las Vegas casino. Beyond that, there’s not much else.

It’s especially tragic that the Cold War enjoys so few monuments, because its physical legacy is the most monumental of all. Consider the Titan I missile complexes: Deployed in the 1960s, these 18 sites were masterpieces of engineering, awe-inspiring in their scale, testaments to the power of collective action on the order of the Egyptian pyramids. Each was the equivalent of an entire town built completely underground, with its own self-sufficient utility systems, designed to launch and withstand multimegaton nuclear bombs. Each required 32,000 cubic yards of concrete and 7,500 tons of steel. They included three enormous silos, a control center, and a powerhouse, all interconnected by a half-mile of tunnels and suspended on vast impact-absorbing spring beds. As military historian David Spires observes, missile propellant was pumped “at temperatures as high as 120 degrees Fahrenheit to as low as -400 degrees, through shock-resistant piping at various flow rates under 6,200 psi of pressure.” Simply maintaining these complexes required procedure manuals that were hundreds of pages in length.

The Cold War produced such modern wonders by the thousand. But our chances for even one real Cold War museum are remote. Hours after the USSR collapsed, conservatives proposed monuments to “America’s victory over communism.” However, their efforts crumbled because of a lack of public support, as the progressive historian Jon Wiener demonstrated in How We Forgot the Cold War. Many citizens, young people especially, doubt the war had a real winner. Gretchen Heefner, who teaches Cold War history at Northeastern University, says her students are “skeptical of American power and governance,” influenced by their perception of the Iraq and Afghanistan wars as misguided.

At this point, Americans don’t even know what the Cold War was. A recent Newsweek quiz asked people to name the war’s target; only a fourth of respondents correctly answered communism. (Common incorrect answers included the Great Depression, slavery, and climate change.) It’s a deeply ironic state of affairs, given that millions of Americans—perhaps even most Americans—could find amazing physical monuments to this chapter in history within an hour’s drive. The only catch: We’re not allowed to see them. And soon many of them will be gone.

For a visceral appreciation of the Cold War’s scale, you must stand before behemoths like Alfa Test Stand I, which rises into view soon after we cross into Santa Susana. Built in 1954 and designed to withstand the force expended by the thrust chambers of Atlases, America’s first intercontinental ballistic missiles, the stand is an imposing tangle of steel rather like a blocky Eiffel Tower. It tapers to a platform 90 feet up, with the whole apparatus perched above a mammoth exhaust chute.

The moon is shrouded in cloud, so the photographers decide to try starting out in Alfa’s underground control room. Last time they couldn’t get in, because it was chained up with a padlock; it would have been short work to break it, but they worried about noise, and besides, they operate by the hiker’s ethic of “leave no trace.” (It’s a wise ethic, legally speaking, since forced entry or lock-pick possession would convert misdemeanor trespassing into a felony.) As we look around, Freskos notices new portable toilets and caution tape nearby—signs that contractors have been working here recently, which is promising news. We descend some stairs, and sure enough, we find another control-room door that isn’t locked. It’s not even shut.

We dead-bolt the door behind us, locking ourselves inside a room that, in both size and decor, roughly approximates an insurance branch office from the 1960s. It feels lived in, as if it had been abandoned just yesterday. Soft-drink cans and business cards are still strewn across the desks, and the place reeks with a sort of industrial must. It’s easy to imagine it full of crew-cut men in short-sleeved shirts and blocky eyewear. For the first time, I truly feel like a trespasser. Haeber, the preservationist, loves this feeling the most—the sense that he’s seeing how the Cold War looked to the men on the ground, the ones who did the drudge work to realize the ambitions of generals and politicians.

Getting to one test stand involves walking along a slick mile-long water pipe as high as 30 feet off the ground.

All three men set to work with their cameras. Shooting the control panel is a particular technical challenge; the room’s lights no longer work, and the panel’s reflective surface makes standard photographer’s rigs a challenge. So they illuminate the scene through what they call “light painting”: leaving their camera shutters open while sweeping flashlights of varying color and intensity over the panel, a bewildering 20-foot-wide array of bristling analog switches. This single shot takes the men more than an hour to refine; they keep accidentally overexposing one section of the panel such that it shows up as a white glare, like a side-view mirror reflecting the sun. “Are we overthinking this?” Haefner asks repeatedly.

The shot finally accomplished, we come back outside to find the moon still hidden behind clouds. Haefner checks his weather app, which promises clear sky an hour from now. “It just needs to get a little colder,” Haeber says, proposing that we walk to Coca, an even larger missile installation at the site. Getting to Coca from Alfa involves walking along a slick mile-long water pipe; in some places it’s as slender as 18 inches in diameter, as high as 30 feet off the ground. Balancing on it isn’t difficult technically, but it’s nevertheless a mental challenge—start to worry and you’ll find yourself wobbling. In the end, I’m the only one to fall: When the pipe gets closer to the ground, I celebrate too soon and slip, crashing down onto the cold metal—but luckily not to the hard earth below.

Along the way, we pass 12 massive water towers, which hold up to a million gallons each. During missile tests, nozzles surrounding the stands sprayed continuously, mostly to cool the equipment and guard against fire. Soon we reach Coca’s two stands, 160-foot-high colossi whose size rivals even the surrounding rock outcrops. Huge platforms extend from the stands like diving boards. “That would be a fun one to rappel off of,” Haeber muses. (Past expeditions have required them to learn technical climbing skills.) The tail chute, where the flames came out, gapes as if frozen in an eternal scream. “It’s so quiet here now, but when this place was going, it was so loud,” Haefner says. In a reminiscence published in an official history, one observer remembered the report of an engine test as “exceeding the loudest rock band you’ve ever heard.”

Eventually the clouds part, and the three scurry like ants up and down the test stands to get the shots they want. Once finished with Coca, they return to Alfa to shoot its exterior. It’s past midnight, we’ve hiked about 10 miles up and down canyons, and only adrenaline keeps us on our feet. They begin another marathon shooting session, interrupted twice when a cruiser pulls up the road, forcing us all to dive out of sight. The second time this happens, I end up pinned behind a wall, inches from the road. Forced to sit still, I can’t help but fall fast asleep.

Haeber, Haefner, and Freskos have made more than a thousand site visits in all. They’ve been caught a handful of times but have always escaped conviction. One of their closest calls came at the Port of Los Angeles, when cops found Haefner exploring a former coal-export terminal. He was cuffed, pushed down into a police cruiser, and interrogated for three hours. Afterward he was issued a citation and even assigned a court date. But the night before court, he phoned the assistant DA, groveled like mad, and got the charges dropped.

Without a doubt, their most foolhardy expedition has been the one inside Vandenberg Air Force Base, the only active site they’ve infiltrated. When Haeber first proposed it, Freskos called the idea “beyond stupid” and refused to go. The base literally has missiles ready to be equipped with nuclear warheads and is guarded under high security. But for missile-testing aficionados, Vandenberg is the equivalent of Disneyland: Thor, Atlas, Titan, Minuteman, Peacekeeper, and Interceptor missiles all have been tested there. When Haefner and Haeber started to consider the idea, they subdivided the infiltration into a series of small steps, each of which they realized was feasible; at that point, they felt they had no choice but to try. It nearly ended in disaster. Haeber suspects that he tripped a sensor; an AR-15-wielding guard drove right up to where he was hiding. “I was literally in his spotlights. I don’t know how he didn’t see me,” Haeber says. “My heart was just pounding.” Fortunately the guard got called away before he could get out to investigate. At another point they had to hop a bridge guardrail and cling to a pipe while a car passed.

Things got scarier when Haeber blogged about the adventure afterward. The pair were pursued by the Department of the Interior and the FBI, whose investigators tried—unsuccessfully—to trick them into admitting when they had infiltrated the base. (To prosecute, they’d need to prove that the statute of limitations hadn’t elapsed.) The Feds came down particularly hard on Haefner, probably because he works for a federal agency. He and Haeber had to hire lawyers at considerable expense and got thoroughly freaked out. Eventually they avoided prosecution but were formally banned from Vandenberg. “Your presence is detrimental to the maintenance of good order and discipline,” a letter to them read.

Nevertheless, they now count Vandenberg among their favorite sites. “We found some pretty amazing things,” Haeber says, “like the launch panels for the Atlas D program. These were mobile. They were just as small as your desktop fax machine.”

The photographers also got into trouble for a blog post about the Mothball Fleet—a collection of retired Navy vessels docked in Suisun Bay, California, which the three infiltrated and photographed over several weekends. (To get there, they inflated a raft, hid in a cove while waiting for security boats to pass, and then listened for encrypted Coast Guard codes on a scanner to elude the regular patrols.) After Haeber recounted this story online, investigators from the Department of Transportation visited Freskos at his workplace, and the other two were grilled by phone.

It appears as if this last trip to Santa Susana won’t end in arrest, but the three remain nervous: “I don’t really breathe until I get into an untowed car,” Haefner says. Security personnel who happen upon suspicious-looking vehicles often just wait there for the owners to return. At the end of an earlier outing, Haeber and I were ambushed by a guard and a state trooper who—after questioning us—ordered us to leave and never return. But this time we’re relieved to find Haefner’s truck right where we left it, undisturbed in the brightening dawn. We change out of our filthy clothes and head to a 24-hour diner.

I ask them why they do it. Why do they risk arrest, endanger their day jobs, and surrender nearly all their nights and weekends?

Haefner’s girlfriend often asks the same thing, he replies with a laugh. But he can’t shake the feeling he gets from these explorations. “It’s surreal, like a waking dream. Space that’s abandoned feels more intensely alone than the wilderness.” Haeber chalks it up to curiosity: “It’s like being a child and wondering what’s beyond the next corner.” Freskos has trouble defining his own motivations: “It’s so intense and hard to explain.”

They won’t divulge which sites they’re contemplating visiting next, at least not on the record. Haeber continually adds to a handcrafted map of 1,500 possibilities, sites he compiles by combing through the Internet, satellite imagery, and news stories. He will divulge the names of some dream destinations, including Portsmouth Naval Prison in Maine (once called the Alcatraz of the East), Building 257 on Plum Island in New York (where scientists secretly researched biological weapons), and the Hanford Site in Washington state (which once produced plutonium). All present huge obstacles, but the three try never to dismiss any site as impossible. The nuclear facility at Oak Ridge, Tennessee, which spends $150 million annually on security, was infiltrated in 2012 by three bolt-cutter-wielding peace activists—an octogenarian nun and her two senior-citizen accomplices.

Beyond the personal thrills, Haeber, Haefner, and Freskos feel their illegal hobby is a vital act of historical preservation. In this they draw inspiration from the WPA photographers who, during the 1930s, captured the few remaining pockets of America still stuck in the 19th century. The federal government does employ a few official documentarians, but they can get to only a small slice of all that needs chronicling.

“Where we are today, as a society, is a product of the Cold War mentality,” Haeber says. “Exploration to me is taking a very local place, a single spot on the landscape, and connecting it to these big issues—national governments, politics, war, defense, consumption, cultural change, religion, societal change, how people interact with one another, why communities are the way they are.” In Haeber’s view, the Cold War’s misguided premise (that one can defeat communist philosophy with nuclear missiles) lives on in the common belief that all problems are best solved by bigger and better technology. Not getting along with the nation next door? Upgrade your arsenal. With your neighbor? Webcam his misdeeds. With your God? Download a scripture-a-day app.

Indeed, the ultimate irony of their hobby is that the danger inherent in it—an omnipresent risk of arrest merely for the act of documenting historical structures, built at staggering taxpayer expense and long ago abandoned, their military purpose exhausted—is itself a holdover of the Cold War mentality, under which even modest transparency about military matters became forbidden in the name of “national security.” Someday the federal government might come to its senses and open up its awe-inspiring Cold War legacy to the people who paid for it. Until then, though, we will have to content ourselves with these photographs.