In 1982 a group of six Parisian teenagers pulled off what would prove to be a fateful theft. Lifting a grate from the street near Eiffel Tower, 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.

The six 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.” The UX members push themselves beyond boundaries. The group claims to have conducted 15 such covert restorations, often in centuries-old spaces, all over Paris.