You're probably familiar with the most famous part of Paris's underground city, and that's because it's equal parts creepy and awesome. The catacombs are the final resting place for between six and seven million people, and to put that in perspective, that's the human being equivalent of, roughly, Rio de Janeiro. The tunnels already existed when they started burying people there, and they date back to around the 13th century. Limestone was quarried from beneath the city to build on top of the ground, and while that doesn't really seem like the best idea, we're not here to judge. In the 18th century, the body count rose, cemeteries overflowed quite literally, and the city started smelling so bad that even the perfume shops started complaining. The bodies were moved underground.
About a mile of the catacombs are open to the public, but there's another part of the underground of the City of Lights—we like to call it the Underdark—that's definitely not as public.
Around 185 miles of tunnels are under Paris, and while you're not technically supposed to be there, people have never let technicalities stop them. In 2004, law enforcement found that someone had set up an underground movie theater in part of the tunnels near the Eiffel Tower, along with a lounge, workshop, dining area, and bar. It's all the work of a group called The Mexican Consolidated Drilling Authority, give or take some translation difficulties. According to their spokesperson, Lazar Kunstmann, they're just one branch of an organization dedicated to mapping and preserving the tunnels … and having a bit of fun. At the time the movie theater was found, Kunstmann went on record saying seven other sites are also deep in the tunnels under Paris, but that's all he'd say. Secret tunnels, a secret society, and code names. It couldn't get any better.