The catacombs of Paris: hundreds of kilometers of labyrinthine tunnels, with the bones of more than six million Parisians from prior centuries. Most of these tunnels were once carrières, or mines for limestone and gypsum. Sections were converted into ossuaires for the overflowing cemeteries in the 18th and 19th centuries. Other tunnels were filles in, and in the 20th century parts of the system were converted into bomb shelters and bunkers. Today, these tunnels still form a network underlying most of the city south of the Seine.