The Catacombes de Paris are an underground ossuary of the city of Paris whose entrance is located near the place Denfert-Rochereau, in the 14th arrondissement. Located 20 meters below the city, this historic underground cemetery is about 1.7 km long and is part of a much wider network of underground passages, about 285 km, passages that were part of a quarry from which in ancient times to from which limestone, gypsum and clay were extracted in Roman times. The history of the catacombs dates back to 1786, when the City of Paris decided to use this underground network as mass graves to cope with the spread of epidemics caused by the saturation of some cemeteries. The catacombs therefore contain the remains of about six million people who were buried here following the closure of the Parisian cemeteries. In the midst of so many common people were also buried men and women of their time illustrious who have marked the history of this city such as Madame de Pompadour, lover of Louis XV, the writer Charles Perrault and various famous victims of the guillotine such as Georges Jacques Danton, Camille Desmoulins and Maximilien de Robespierre. From the 19th century it became a tourist attraction and was thus opened to the public in 1874. During the Second World War the network of tunnels was used by the French Resistance to fight the Nazi occupation.