Mines Of Tunnel Network, Catacombes De Paris, Paris, France

The Left Bank rests upon rich Lutetian limestone deposits. This stone built much of the city, but it was extracted in suburban locations away from any habitation. Because of the post 12th-century haphazard mining technique of digging wells down to the deposit and extracting it horizontally along the vein until depletion, many of these (often illicit) mines were uncharted, and when depleted, often abandoned and forgotten. Paris had annexed its suburbs many times over the centuries, and by the 18th century many of its arrondissements (administrative districts) were or included previously mined territories.
The undermined state of the Left Bank was known to architects as early as the early 17th-century construction of the Val-de-Grâce hospital (most of its building expenses were sunk into its foundations), but a series of mine cave-ins beginning in 1774 with the collapse of a house along the "rue d'Enfer" (near today's crossing of the Avenue Denfert-Rochereau and the boulevard Saint-Michel) led King Louis XVI to name a commission to investigate the state of the Parisian underground. This led to the creation of the inspection Générale des Carrières (Inspection of Mines) service.
• Establishment of the ossuary
The need to eliminate Les Innocents gained urgency from May 30, 1780, when a basement wall in a property adjoining the cemetery gave way under the weight of the mass grave behind it. The cemetery was closed to the public and all intra muros (Latin: "within the city walls") burials were forbidden after 1780. The problem of what to do with the remains crowding intra muros cemeteries was still unresolved.