In the 1770s the Paris authorities was afraid that the corpses at the cemetaries could be a source of infection and polution - maybe rightfully. It was decided to move all the corpses to a suitable place, and this was the catacombs. This process continued until 1860. In the catacombs you can walk kilometres between walls of skulls and bones. Behind the walls are a lot more skeletons. Read more at wikipedia.