Catacombs

In the 18th century, Paris decided they were running out of places to build, so they emptied out all of the cemeteries and stacked the bones below the surface of the earth in the catacombs. The catacombs themsevles are pretty increadible... basically, way back in Pangea days, France was underwater. The ground below the dirt is all ocean bed. There are even little tiny crustacious shells and fossils. Below Paris now they have mines out all these tunnels... which were turned into the catacombs, and also used for other things like growing mushrooms and potatoes and endives. The large blocks that they cut out to make these tunnels are the stones used to build the city itself, like the churches and castles etc.

Before you enter the catacombs there is a sign that says in latin "You are now entering the Empire of death." then you walk through tunnel after tunnel of stacked bones of old aristocratic Parisians.