Beneath the streets of Paris lie more than 280 kilometers of underground spaces, including tunnels, empty chambers, and old stone quarries. These subterranean areas have been mapped, reinforced, and monitored continuously since 1777 by the Inspection Générale des Carrières (IGC), a French state service. The IGC was established by a decree from King Louis XVI’s council in response to a series of dangerous sinkholes and building collapses. Interestingly, the very stone used to construct Paris—its famous Haussmannian buildings, churches, and monuments—was extracted from these underground quarries. Over centuries, quarrying created vast voids beneath the city, turning Paris into a kind of "cheese grater" that shaped its surface grandeur. The urgency of the IGC’s mission became clear almost immediately after its creation. On April 4, 1777, the same day the service was established, a house on Rue d'Enfer collapsed into an old quarry 40 meters below. This was one of several dramatic collapses between 1774 and 1776, including a major incident in December 1774 that swallowed several buildings thirty meters deep. These events led to a citywide inspection of all known underground excavations, revealing that many public roads and structures in southern Paris were at risk of collapse. The IGC was tasked with mapping these spaces and ensuring public safety. Charles-Axel Guillaumot, the first general inspector of the IGC, led the service from its founding until his death in 1807. His work included exploring underground galleries, reinforcing unstable areas with masonry pillars, and creating detailed records of all known voids. Today, the IGC operates under the City of Paris, as part of the Direction of Road and Mobility, and its responsibilities extend beyond the city to include surrounding areas like the Hauts-de-Seine and Seine-Saint-Denis. The service maintains a comprehensive atlas of the underground, consisting of 458 detailed maps, geological surveys, and databases that have been growing for over 200 years. The risk of subsidence in Paris varies due to the different types of rock beneath the city. In the south, Lutetian limestone is more stable, while in the north and northeast, soluble gypsum rock can dissolve over time, creating large cavities. Even minor leaks from underground pipelines can cause subsidence in these areas. Montmartre, for example, has experienced past collapses linked to gypsum dissolution. Since 1980, the IGC has been injecting concrete into these unstable areas to prevent further damage. However, some risks still remain. It's important to note that the underground consists of three distinct parts: the extensive quarry network, the official catacombs, and restricted areas. The catacombs, located in the 14th arrondissement, are a public ossuary where millions of Parisians are buried. The rest of the underground remains closed to the public, and unauthorized exploration—known as cataphilia—can be dangerous. As the IGC continues to fill voids with concrete, it gradually reduces the accessible space for both explorers and inspectors.