Catacombs of Paris ("Catacombes de Paris"), about 20 meters (60 feet) underground, under Paris city center.
This is a part of the 200 Km (130 Miles) network of ancient tunnels not opened to the public, but regularly visited by trespassing urban cavers called "cataphiles".
This is one of my favorite playgrounds, but it's also a dangerous and forbidden place where you can easiely get lost.
You can look at several interresting videos of the Catacombs here (videos are in french).
Read this excellent article (in english) on National Geographic about this amazing off-limit underground world.
Those interconnected caves and tunnels are the remnent of antique limestone quarries were the stone was extracted to build the houses of Paris. Part of them are now used to store human bones moved from various old cemeteries around 1790, hence the (incorrect) name "Catacombs".
Only a small section of the Catacombs, called the Ossuary, is opened to the public near Place Denfer-Rochereau in the 14th arrondissement of Paris. Visitors can see the the bones of about 6 millions people filling up large underground chambers. See Les "Catacombes de Paris" (official website) for more infos.