In the catacombes of Paris, there are about 6 million skeletons. That is 3 skeletons for each inhabitant of the city of Paris.
The catacombes are old corridors created from extracting limestone blocks. In the 18th century, there was serious submergence due to collapsing of these corridors.
The main graveyard of Paris, Cimetière des Innocents, got seriously overcrowded in the 18th century. All of the bodies of Cimetière des Innocents were exhumed and put in these corridors to support them.