Beneath the streets of Paris, from the Avenue Denfert-Rochereau to the Place Saint-Michel and the Montsouris Park, the ground is not solid. Instead, it contains a vast network of underground galleries stretching over nearly 300 kilometers. This network is a remnant of centuries of mining activity that once supplied materials for the city's construction. The mining began long before the 18th century, with excavations initially taking place in open areas and later moving underground from the 12th to the 13th century. A particular type of stone, known as Lutetian limestone or "coarse" limestone, was especially valuable, used in the construction of landmarks like Notre-Dame. In the north, gypsum was extracted to make plaster. As the city grew, it left behind vast underground voids that were never mapped or properly documented. The dangers of these unrecorded quarries became evident on December 17, 1774, when a section of the Rue d'Enfer (now Avenue Denfert-Rochereau) collapsed into an old underground quarry, swallowing buildings thirty meters below. A similar incident occurred in 1777, on the same day the first inspector was appointed, when a house vanished into a 40-meter-deep quarry. These events highlighted the risks of unmanaged underground spaces. In response, King Louis XVI established the General Inspection of Quarries of Paris (IGC) in 1777. The IGC was tasked with mapping and stabilizing the abandoned quarries, ensuring the safety of the city above. This was a monumental task, expected to take decades rather than months, and it laid the foundation for ongoing underground monitoring in Paris. At the same time, Paris faced another pressing issue: its cemeteries were overflowing. The oldest and largest, the Cemetery of the Saints-Innocents, had been in use for nearly a thousand years and was finally closed in 1780. The city needed a new place to store the remains, and the solution was the abandoned quarries in the plain of Montrouge, known as la Tombe-Issoire. In a nightly operation, bones were removed from the cemeteries, cleaned, and transported in covered wagons to the quarries. This process continued for fifteen months, with over six million bones eventually moved from seventeen cemeteries, monasteries, and places of worship by 1861. Today, the official part of the catacombs open to the public is only a small portion of the entire underground network, covering just 1.7 kilometers out of nearly 300 kilometers of tunnels. Two and a half centuries after its creation, the IGC continues to operate, now under the administration of the City of Paris and covering surrounding areas. Its mission remains unchanged: to map, inspect, and stabilize the underground quarries to protect the city and its residents. The IGC employs a team of engineers, geologists, surveyors, and technicians who work to fill or reinforce the tunnels as needed. While some sections of the network remain fragile, the work of consolidation has continued without pause. The IGC also addresses risks such as ground movements from old quarries and the dissolution of pre-Ludian gypsum, as well as flood prevention due to rising water tables. Even today, field teams decide, gallery by gallery, which parts of the underground can remain untouched and which must be filled to ensure the safety of the city above.