In 1785, it was decided to solve the hygienic and aesthetic problems posed by the city's overflowing cemeteries by exhuming the bones and storing them in the tunnels of three disused quarries. One ossuary so created is the Catacombes, without a doubt Paris' most viscerally disturbing tourist site. After descending 20m (65ft) below street level, visitors can shudder through corridors filled with intermingled rib cages, bones and skulls that have been neatly stacked along the walls. People over 60 can get in for free, which says a lot about the French sense of humour. The tunnels, which were used by the Résistance during WWII as a headquarters, are south of the Seine.

This underground maze consists in more than 300 km of galleries under the city of Paris distributed between several networks.  The underground catacombes or quarries spreadunder the 5th, 6th, 8th, 12th, 13th, 14th, 15th, 16th and 20th districts. The most extensive network, located under the 5th, 6th and 14th districts, is about one hundred kilometers long.

A famous architect left his mark in the quarries: Mansart.  He was charged by the queen, Anne d'Autriche, to build a church and an abbey (to thank God for the birth of her son, the future Louis XIV) that has since been named  Val-de-Grâce. While digging to establish the foundations of the future building, he discovered incredible emptinesses in the subsoil and had to strengthen it before building anything. This work, given its importance, took time and money.  Wondering about the slow advancement of the work, the queen fired Mansart, just after he finished the construction of the first floor.  It is the reason why some people say that the most beautiful construction of Mansart at the Val-de-Grâce is under ground.