Skulls in the Catacombs.

Southern Paris contains a mass underground grave system. This dates back to a plague in the 1800's wherein the city ran out of cemetery space. They actually had carts and priests that drove around the city picking up bodies and administering the last rites. And they stacked all the bodies underground. Now you can take a tour and see all the bones. It was cool - at one point a little French kid jumped out of the shadows though, and I screamed!