Catacombs of Paris. Behind the assembled piles of skulls, tibias, and femurs in the Paris catacombs is a chaos of bones. In the 18th and 19th centuries the city dug up an incredible number of skeletons from over-flowing cemeteries and added them at night into old quarries.
http://www.flickr.com/photos/albany_tim/ / CC BY 2.0 For urban explorers, few places are more coveted than the Catacombs of Paris. Long sealed off to the public, these harrowing subterranean chambers house an 18th century cemetery – essentially an anonymous mass grave. Beyond this, a labyrinth of tunnels extends for 280km beneath Paris in a network of disused stone mines and galleries.