The UX (for Urban eXperiment) is an underground organization that improves hidden corners of Paris. Their works have included restoring the Pantheon clock, building a cinema, complete with bar and restaurant, underneath the Trocadéro, restoring medieval crypts, and staging plays and readings in monuments after dark. The group's membership is largely secret, but its spokespeople include Lazar Kunstmann.
Beginning in September 1981, the founders of the group stole plans of the many underground passageways and tunnels for which Paris is famous. Using these information venues as a base,  the group of anonymous artists and citizens has restored much of the underground infrastructure, including the restoration of the Pantheon clock which chimed for the first time in many years after their repair. The shadowy group of unknowns is also responsible for over a dozen other projects, including those which the French government has not chosen to do or for which they lack funds.
The organization is divided into teams: an all-female team (the Mouse House) specializing in infiltration, a team running an internal messaging system and coded radio network, a team providing a database, a team organizing underground shows, a team doing photography, and a team (Untergunther) doing restoration.
Untergunther's membership includes architects and historians. In October 2007, they received attention for a project, assisted by professional clockmaker Jean-Baptiste Viot, to clandestinely restore the famous clock in the Panthéon. Never caught, upon completion they announced their work at a meeting with the administrator of the Panthéon, who called the police.
La Mexicaine De Perforation (The Mexican Consolidated Drilling Authority) is another subdivision of the UX which provides clandestine artistic events.
In September 2004, French police discovered an underground movie theatre run by La Mexicaine De Perforation. The makeshift theatre contained a movie screen, a well stocked bar, and a kitchen. Telephones and electricity were brought in from an unknown location. Movie titles ranging from 1950s classics to modern thrillers were also discovered. When the police returned for a formal investigation, all the equipment had disappeared—all that was left was a note on the floor reading, "Ne cherchez pas" ("Do not search").
The Parisian authorities oppose the group's actions, starting a police unit to track them through the sewers and catacombs of Paris, and attempting to apprehend and charge them.
Charges were brought against the four Untergunther restorers of the Pantheon clock, but at trial, after 20 minutes' deliberation, the judge ruled in their favor. One of the government's prosecutors referred to the charges as "stupid".
- "Undercover restorers fix Paris landmark's clock", The Guardian, 26 Nov 2007
- Lackman, Jon (January 20, 2012). "The New French Hacker-Artist Underground". Wired. Retrieved January 25, 2012.
- Lackman, Jon (January 20, 2012). "The New French Hacker-Artist Underground". Wired.
- "All". Wired. January 2012.
- Henley, Jon (September 11, 2004). "Paris's new slant on underground movies: Clandestine group reveals how it built its cinema beneath the city". The Guardian. Manchester: Guardian Media Group. Retrieved April 24, 2011.
There are, at most, 15 of them. Their ages range from 19 to 42, their professions from nurse to window dresser, mason to film director. And in a cave beneath the streets of Paris, they built a subterranean cinema whose discovery this week sent the city's police into a frenzy... Until recently very few people did have a clue about La Mexicaine de la Perforation, a clandestine cell of "urban explorers" which claims its mission is to "reclaim and transform disused city spaces for the creation of zones of expression for free and independent art".
- Lackman, Jon (January 20, 2012). "The New French Hacker-Artist Underground". Wired. Retrieved February 11, 2012.
- "Urban eXperiment, the book by Lazar Kunstmann, the spokesman for the UX"
- "Underground ‘terrorists’ with a mission to save city’s neglected heritage", The Times, 29 Sep 2007
- Untergunther web site
- Pantheon, user's guide - The movie about the Untergunther and La Mexicaine De Perforation
- The New French Hacker-Artist Underground
- "Interview with Lazar Kunstman about the underground cinema project"
- Underground cinema programming guide
- "UnterGunther: French Urban Explorers Sneak Into Pantheon For A Year, Repair 150-yo Clock", greg.org