The Catacombs, which form a veritable labyrinth beneath the very heart of Paris, were created in the galleries of the former quarries whose stone was used to build the capital. Situated twenty metres below ground, the ossuary contains the remains of millions of Parisians, transferred there gradually between the late eighteenth and mid-nineteenth centuries as graveyards were being closed because of the risk they posed to public health. The first of these was the cimetière des Innocents graveyard in 1786 in what is now the district of Les Halle. In the long maze of dark galleries and narrow passages, visitors can see a tableau of death with bones arranged in a macabre display of high Romantic taste. The alexandrine verse "Arrête, c'est ici l'empire de la mort" [Halt, this is the realm of Death ] above the entrance to the ossuary is just one of an extensive series of maxims, poems and other sacred and profane passages giving pause for thought during the tour. This unusual site movingly brings the history of the Parisian people back to life and takes visitors on a timeless journey. The Catacombs represent the interface between the history of Paris and the Earth's geological evolution. Forty-five million years ago, Paris and the surrounding area were covered by a tropical sea. Dozens of metres of sediment accumulated on the sea bed, forming over lime the limestone deposits visible in the Catacombs today. Geologists worldwide call this period in the history of the world the Lutetian period, alter Lutetia, the Gallo-Roman name for Paris.

http://www.catacombes.paris.fr/en/catacombs/more-2000-years-history