Thirty years ago, in the dead of night, a group of six Parisian teenagers pulled off what would prove to be a fateful theft. 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.

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.

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.