The underground area of the three Parisian hills (Montparnasse, Montrouge, Montsouris) has been exploited since Gallo-Roman times. From 1785 to 1814, the uninhabited sections were used as charnel houses: several million skeletons, particularly from the Cemetery of the Innocents were transferred there. In the galleries, the remains laid against the walls provide macabre decoration. In August 1944, the headquarters of the Resistance were set up here.