Thirty years ago, in the dead of night, a group of six Parisian teenagers pulled off what would prove to be a fateful theft. They met up at a small café near the Eiffel Tower to review their plans—again—before heading out into the dark. Lifting a grate from the street, they descended a ladder to a tunnel, an unlit concrete passageway carrying a cable off into the void. They followed the cable to its source: the basement of the ministry of telecommunications. Horizontal bars blocked their way, but the skinny teens all managed to wedge themselves through and ascend to the building’s ground floor. There they found three key rings in the security office and a logbook indicating that the guards were on their rounds.

But the guards were nowhere to be seen. The six interlopers combed the building for hours, encountering no one, until they found what they were looking for at the bottom of a desk drawer—maps of the ministry’s citywide network of tunnels. They took one copy of each map, then returned the keys to the security office. Heaving the ministry’s grand front door ajar, they peeked outside; no police, no passersby, no problem. They exited onto the empty Avenue de Ségur and walked home as the sun rose. The mission had been so easy that one of the youths, Natacha, seriously asked herself if she had dreamed it. No, she concluded: “In a dream, it would have been more complicated.”

This stealthy undertaking was not an act of robbery or espionage but rather a crucial operation in what would become an association called UX, for “Urban eXperiment.” UX is sort of like an artist’s collective, but far from being avant-garde—confronting audiences by pushing the boundaries of the new—its only audience is itself. More surprising still, its work is often radically conservative, intemperate in its devotion to the old. Through meticulous infiltration, UX members have carried out shocking acts of cultural preservation and repair, with an ethos of “restoring those invisible parts of our patrimony that the government has abandoned or doesn’t have the means to maintain.” The group claims to have conducted 15 such covert restorations, often in centuries-old spaces, all over Paris.

What has made much of this work possible is UX’s mastery, established 30 years ago and refined since, of the city’s network of underground passageways—hundreds of miles of interconnected telecom, electricity, and water tunnels, sewers, catacombs, subways, and centuries-old quarries. Like computer hackers who crack digital networks and surreptitiously take control of key machines, members of UX carry out clandestine missions throughout Paris’ supposedly secure underground tunnels and rooms. The group routinely uses the tunnels to access restoration sites and stage film festivals, for example, in the disused basements of government buildings.

UX’s most sensational caper (to be revealed so far, at least) was completed in 2006. A cadre spent months infiltrating the Pantheon, the grand structure in Paris that houses the remains of France’s most cherished citizens. Eight restorers built their own secret workshop in a storeroom, which they wired for electricity and Internet access and outfitted with armchairs, tools, a fridge, and a hot plate. During the course of a year, they painstakingly restored the Pantheon’s 19th- century clock, which had not chimed since the 1960s. Those in the neighborhood must have been shocked to hear the clock sound for the first time in decades: the hour, the half hour, the quarter hour.

Eight years ago, the French government didn’t know UX existed. When their exploits first trickled out into the press, the group’s members were deemed by some to be dangerous outlaws, thieves, even potential inspiration for terrorists. Still, a few officials can’t conceal their admiration. Mention UX to Sylvie Gautron of the Paris police—her specialty is monitoring the city’s old quarries—and she breaks into a wide smile. In an era when ubiquitous GPS and microprecise mapping threaten to squeeze all the mystery from our great world cities, UX seems to know, and indeed to own, a whole other, deeper, hidden layer of Paris. It claims the entire city, above- and belowground, as its canvas; its members say they can access every last government building, every narrow telecom tunnel. Does Gautron believe this? “It’s possible,” she says. “Everything they do is very intense.”

It is not at all hard to steal a Picasso, Lazar Kunstmann tells me. One of UX’s early members and the group’s unofficial spokesman, Kunstmann—the name is almost certainly a pseudonym, given its superhero-like German meaning, “Art-man”—is fortyish, bald, black-clad, warm, and witty. We’re sitting in the back room of a student café, downing espressos and discussing the spectacular theft in May 2010 of 100 million euros’ worth of paintings from the Museum of Modern Art of the City of Paris. He disputes the contention of a police spokesperson that this was a sophisticated operation. According to an article published in Le Monde, a solitary individual unscrewed a window frame at 3:50 am, cut a padlock from a gate, and strode through the galleries lifting one work each by Léger, Braque, Matisse, Modigliani, and Picasso. “The thief was perfectly informed,” the officer told the newspaper. If he hadn’t known the window had a vibration detector, he would’ve just broken it. If he hadn’t known the alarm and part of the security system were broken, he wouldn’t have wandered throughout the museum. If he hadn’t known the schedule of night rounds, he wouldn’t have arrived in the middle of the longest quiet period.

Impressive, right? No, Kunstmann says. “He ascertained that nothing was working,” Kunstmann sighs, knowing full well the shoddy security of the museum in question. “The exterior is full of graffiti artists, the homeless, and crack smokers,” he goes on. This would have made it easy for the thief to blend in and surreptitiously watch the windows all night, observing how the guards circulated.

A serious thief, Kunstmann says, would have taken an entirely different approach. In the same building, a sprawling and grand old structure called the Palais de Tokyo, is a restaurant that stays open until midnight. An intelligent thief would order a coffee there and then wander off through the building. “Lots of things have alarms,” Kunstmann goes on. “But you try to set them off and they don’t sound! Why? Because they don’t get turned on until 2 am.” (The museum claims that the alarms work 24 hours a day.) Moreover, there are whole stretches of wall where all that separates the museum from the rest of the building is a flimsy drywall partition. “You just—” Kunstmann makes a punching motion with his hand. “If the guy had been at all professional, that’s what he would have done.”

UX has made a study of museum security, in keeping with its concern for Paris’ vulnerable treasures—a concern not always shared by the city’s major cultural institutions. Once, after a UX member discovered appalling security lapses in a major museum, she wrote a memo detailing them—and left it, in the middle of the night, on the desk of the security director. Rather than fix the problems, the director went to the police, demanding they press charges against the perpetrators. (The police declined, though they did tell UX to cool it.) Kunstmann feels sure that nothing has changed since the break-in at the Museum of Modern Art; the security remains just as subpar as ever, he says.

Kunstmann has a gloomy view of contemporary civilization, and in his eyes this affair illustrates many of its worst faults—its fatalism, complacency, ignorance, parochialism, and negligence. French officials, he says, bother to protect and restore only the patrimony adored by millions—the Louvre, for example. Lesser-known sites are neglected, and if they happen to be out of public view—underground, say—they disintegrate totally, even when all that’s needed is a hundred-dollar leak repair. UX tends the black sheep: the odd, the unloved, the forgotten artifacts of French civilization.

It’s difficult, though, to give an accounting of just how extensive those labors of love have been: The group cherishes its secrecy, and its known successes have been revealed only inadvertently. The public learned of the group’s underground cinema after a member’s bitter ex-girlfriend told the police. Reporters caught wind of the Pantheon operation because UX members erred in supposing they could safely invite the building’s director to maintain his newly fixed clock (more on that later). In general, UX sees communicating with outsiders as perilous and unrewarding. Kunstmann does tell me a story from a recent job, but even that is shrouded in misdirection. Some members had just infiltrated a public building when they noticed kids horsing around on the scaffolding at a construction site across the street, climbing through open windows, and doing dangerous stunts on the roof. Pretending to be a neighbor, one member phoned the foreman to warn him but was chagrined at the response: “Instead of saying, ‘Thanks, I guess I’ll close the windows,’ the guy says, ‘What the fuck do I care?'”

An outsider might wonder whether the teens who founded UX were really so different from those thrill seekers across the street today. Would they rat out their former selves? But when UX members risk arrest, they do so with a rigorous, almost scientific attitude toward the various crafts they aim to preserve and extend. Their approach is to explore and experiment all through the city. Based on members’ interests, UX has developed a cellular structure, with subgroups specializing in cartography, infiltration, tunneling, masonry, internal communications, archiving, restoration, and cultural programming. Its 100-odd members are free to change roles and are given access to all tools at the group’s disposal. There is no manifesto, no charter, no bylaws—save that all members preserve its secrecy. Membership is by invitation only; when the group notices people already engaged in UX-like activities, it initiates a discussion about joining forces. While there is no membership fee, members contribute what they can to projects.

I can’t help but ask: Did UX steal the paintings from the Museum of Modern Art? Wouldn’t that be the perfect way to alert the French to the appalling job their government does protecting national treasures? Kunstmann denies it with a convincing curtness. “That,” he says, “is not our style.”

The first experiment by UX, in September 1981, was an accidental one. A Parisian middle schooler named Andrei was trying to impress a couple of older classmates, boasting that he and his friend Peter often snuck into places and were about to hit the Pantheon, an enormous former church that towers over the fifth arrondissement. Andrei got in so deep with his boast that to save face he had to follow through—with his new friends in tow. Like Claudia and Jamie in that famous children’s book From the Mixed-Up Files of Mrs. Basil E. Frankweiler, they hid out inside the building until it closed. Their nocturnal occupation turned out to be shockingly easy—they encountered no guards or alarms—and the experience electrified them. They thought: What else could we do?

Kunstmann, a classmate of Andrei and Peter’s, joined the group early on. They quickly branched out from mere infiltration. Obtaining the tunnel maps from the ministry of telecommunications and other sources greatly expanded their access. Many Parisian buildings connect to these passages through their basements, which are as badly secured as the tunnels themselves. Most officials, Kunstmann says, act as if they believe in this absurd principle: Tunnel access is forbidden, thus people don’t go there. This, he adds sardonically, is “a flawless conclusion—and what’s more, a very practical one, because if people don’t go there, then it’s unnecessary to do more than lock the entrances.”

It wasn’t until I went down into the tunnels myself—which is illegal and punishable by a fine of up to 60 euros, though explorers rarely get caught—that I understood why French officials are so complacent. Finding an unlocked entrance, without UX’s know-how, required a 45-minute walk from the nearest subway. UX has access to dry and spacious tunnel networks, but the more easily entered ones that I traveled that day were often tiny and half-flooded. By the time I’d retraced my steps, I was exhausted, filthy, and bleeding all over from scrapes.

In some places, UX has been able to create covert connections between networks, using (among other tricks) an invention they call the rolling basin. This is a passage in the bottom of a tunnel that appears to be a grate with water under it; in fact, both grate and water are part of a movable tray on rollers. Voilà—a trapdoor to another tunnel in a different network. The tray itself is made of concrete, so even if someone raps it with a stick, it sounds solid. Kunstmann says UX has a certain weakness for such contrivances but will never possess enough time and cash to build them as extensively as he’d like. “If tomorrow everyone in UX became billionaires, we’d set dues at a billion euros,” he jokes. (But, he adds, “we’ll never be billionaires, because we’re working as little as possible so we can spend as much time as possible on UX.”)

So what does the group do with all this access? Among other things, it has mounted numerous clandestine theater productions and film festivals. On a typical festival evening, they screen at least two films that they feel share a nonobvious yet provocative connection. They don’t explain the connection, leaving it up to the audience to try to discover it. One summer, the group mounted a film festival devoted to the theme of “urban deserts”—the forgotten and underutilized spaces in a city. They naturally decided the ideal venue for such a festival would be in just such an abandoned site. They chose a room beneath the Palais de Chaillot they’d long known of and enjoyed unlimited access to. The building was then home to Paris’ famous Cinémathèque Française, making it doubly appropriate. They set up a bar, a dining room, a series of salons, and a small screening room that accommodated 20 viewers, and they held festivals there every summer for years. “Every neighborhood cinema should look like that,” Kunstmann says.

The restoration of the Pantheon clock was carried out by a UX subgroup called Untergunther, whose members are devoted specifically to restoration. The Pantheon was a particularly resonant choice of site, since it’s where UX began, and the group had surreptitiously screened films, exhibited art, and mounted plays there. During one such event in 2005, UX cofounder Jean-Baptiste Viot (one of the few members who uses his real name) took a close look at the building’s defunct Wagner clock—an engineering marvel from the 19th century that replaced an earlier timepiece. (Records indicate the building had a clock as far back as 1790.)

Viot had admired the Wagner ever since he first visited the building. He had meanwhile become a professional horologist working for the elite firm Breguet. That September, Viot persuaded seven other UX members to join him in repairing the clock. They’d been contemplating the project for years, but now it seemed urgent: Oxidation had so crippled the works that they would soon become impossible to fix without re-creating, rather than restoring, almost every part. “That wouldn’t be a restored clock, but a facsimile,” Kunstmann says. As the project began, it took on an almost mystical significance for the team. Paris, as they saw it, was the center of France and was once the center of Western civilization; the Latin Quarter was Paris’ historic intellectual center; the Pantheon stands in the Latin Quarter and is dedicated to the great men of French history, many of whose remains are housed within; and in its interior lay a clock, beating like a heart, until it suddenly was silenced. Untergunther wanted to restart the heart of the world. The eight shifted all their free time to the project.

They first established a workshop high up in the building, just below its dome, on a floor where no one (including guards) ever went anymore—”a sort of floating space,” as Kunstmann describes the room, punctuated by narrow slits for windows. “It looked down on all of Paris from a height of 15 stories. From the outside it resembled a kind of flying saucer; from the inside, a bunker.” The workshop was outfitted with eight overstuffed armchairs, a table, bookshelves, a minibar, and red velvet drapes to moderate the ambient temperature. “Every element had been conceived to fold up into wooden crates, like the ones visible throughout the monument,” Kunstmann says. In the dead of night, they climbed endless stairs, hauling up the lumber, drills, saws, clock repair equipment, and everything else required. They updated the workshop’s outdated electrical wiring. They spent 4,000 euros on materials, in all, out of their own pockets. On the terrace outside they set up a vegetable garden.

Like at the Museum of Modern Art, where a thief made off with millions in precious art with shocking ease, security at the Pantheon was slipshod. “No one, neither police nor passersby, worried over people entering and leaving the Pantheon by the front door,” Kunstmann says. Nevertheless, the eight equipped themselves with official-looking fake badges. Each had a photograph, a microchip, a hologram of the monument, and a barcode that was “totally useless but impressive,” Kunstmann says. Only very rarely did passing policemen ask questions. At most, it went something like this:

“You’re working at night? Can we see your badges?”

“Here.”

“OK, thanks.”

Once the workshop was complete and thoroughly cleaned, the eight got to work. The first step was to understand how the clock had gotten so degraded—”a sort of autopsy,” Kunstmann says. What they discovered looked like sabotage. It appeared that someone, presumably a Pantheon employee tired of winding the clock once a week, had bludgeoned the escape wheel with an iron bar.

They brought the clock’s mechanism up to the workshop. Viot trained the group in clock repair. First, they cleaned it with what’s called the clockmaker’s bath. This started with 3 liters of water carried up from the public bathrooms on the ground floor. To that was added 500 grams of soft, highly soluble soap, 25 centiliters of ammonia, and 1 tablespoon of oxalic acid—all mixed at a temperature of more than 280 degrees Fahrenheit. With this solution, the group scrubbed and polished every surface. Then they repaired the mechanism’s glass cabinet, replaced broken pulleys and cables, and re-created from scratch the sabotaged escape wheel (a toothed wheel that manages the clock’s rotation) and missing parts like the pendulum bob.

As soon as it was done, in late summer 2006, UX told the Pantheon about the successful operation. They figured the administration would happily take credit for the restoration itself and that the staff would take over the job of maintaining the clock. They notified the director, Bernard Jeannot, by phone, then offered to elaborate in person. Four of them came—two men and two women, including Kunstmann and the restoration group’s leader, a woman in her forties who works as a photographer—and were startled when Jeannot refused to believe their story. They were even more shocked when, after they showed him their workshop (“I think I need to sit down,” he murmured), the administration later decided to sue UX, at one point seeking up to a year of jail time and 48,300 euros in damages. Jeannot’s then-deputy, Pascal Monnet, is now the Pantheon’s director, and he has gone so far as to hire a clockmaker to restore the clock to its previous condition by resabotaging it. But the clockmaker refused to do more than disengage a part—the escape wheel, the very part that had been sabotaged the first time. UX slipped in shortly thereafter to take the wheel into its own possession, for safekeeping, in the hope that someday a more enlightened administration will welcome its return.

Meanwhile, the government lost its lawsuit. It filed another, which it also lost. There is no law in France, it turns out, against the improvement of clocks. In court, one prosecutor characterized her own government’s charges against Untergunther as “stupid.” But the clock is still immobile today, its hands frozen at 10:51.

The members of UX are not rebels, subversives, guerrillas, or freedom fighters, let alone terrorists. They didn’t repair the clock to embarrass the state, nor do they entertain dreams of overthrowing it. Everything they do is intended for their own consumption; indeed, if they can be accused of anything, it’s narcissism. The group is partly responsible for the fact that it is misunderstood. Its members acknowledge that most of its external communications are intended as misdirection—a way to discourage public officials or others from meddling in its operations. They try to hide themselves within the larger mass of Parisians who venture into the city’s recesses simply as partiers or tourists.

Why do they care about these places? Kunstmann answers this question with questions of his own. “Do you have plants in your home?” he asks impatiently. “Do you water them every day? Why do you water them? Because,” he goes on, “otherwise they’re ratty little dead things.” That’s why these forgotten cultural icons are important—”because we have access to them, we see them.” Their goal, he says, isn’t necessarily to make all these things function once again. “If we restore a bomb shelter, we’re certainly not hoping for new bombardments so people can go use it again. If we restore an early 20th-century subway station, we don’t imagine Electricité de France will ask us to transform 200,000 volts to 20,000. No, we just want to get as close as possible to a functioning state.”

UX has a simple reason for keeping the sites a secret even after it has finished restoring them: The same anonymity that originally deprived them of caretakers “is paradoxically what’s going to protect them afterward” from looters and graffiti, Kunstmann says. They know they’ll never get to the vast majority of interesting sites that need restoration. Yet, “despite all that, the satisfaction of knowing that some, maybe a tiny fraction, won’t disappear because we’ll have been able to restore them is an extremely great satisfaction.”

I ask him to elaborate on their choice of projects. “We can say very little,” he replies, “because to describe the sites even a bit can give away their location.” That said, one site is “belowground, in the south of Paris, not very far from here. It was discovered relatively recently but elicited very strong interest. It totally contradicts the history of the building above it. In examining what’s belowground, one notices that it doesn’t correspond to the information one can obtain about the history of the site. It’s history in reverse, in a way; the site was dedicated to an activity, structures were placed there, but in fact the site had been dedicated to this activity for quite a long time.”

Walking across the Latin Quarter alone on a balmy evening, I try to guess what site Kunstmann is describing, and the city transforms before my eyes, below my feet. Did counterfeiters once operate out of the basement of the Paris Mint? Was the Saint-Sulpice church founded on the site of an underground pagan temple? Suddenly, all of Paris seems ripe with possibility: Every keyhole a peephole, every tunnel a passageway, every darkened building a theater.

But it’s also clear that UX retains its love affair with its first and best canvas, the Pantheon. While this story was closing, a colleague needed to reach Kunstmann about a fact-checking question. Kunstmann had told her to call “any time,” so even though it was 1 am in Paris, she rang. When he picked up the phone, he was panting—from moving a couch, he said. She asked her question: When the clock had stopped chiming after the repair, what time remained frozen on its face? As it happened, Kunstmann was in the Pantheon at that very moment. “Hold on,” he said. “I’ll look.”

Jon Lackman (jonlackman.com) is a journalist and art historian.