The Catacombs, Paris.
This part of the network of tunnels under the city holds bones moved from overcrowded cemeteries in the late 1700's. A much larger sprawl of abandoned quarries and other tunnels goes on for miles but is officially off-limits to the public.
More info: Wikipedia / Infiltration: Paris Underground / The secret movie theater / Catacombes.info (French) / Don't get lost. See also 2004.03.22.