Exploring Paris Underground

Unbeknownst to most, an infinite number of underground structures lie beneath Paris. There are, of course, the metro, the sewer system, technical access passages, and former passive defense fortifications, but there is also a vast network of quarries*, with several hundred kilometers of tunnels and galleries. Residents started digging these galleries at least as far back as when Julius Ceasar referred to the city as Lutetia. To index, survey, and maintain this vast network, the City of Paris created a specific department called the “Inspection Générale des Carrières” (the Department of Mining and Underground Works). To learn more about the key role the IGC plays for the city, we met with Jules Querleux, one of the department’s engineers who spends most of his days working underground.

Inspection générale des Carrières (IGC)

The IGC (Department of Mining and Underground Works) is one of the City of Paris’ many service departments. Created in 1777 following the collapse of several streets throughout the city, the department is in charge of managing the risks related to ground subsidence in Paris and in several adjacent municipalities. Day to day, the IGC maps, conducts geotechnical, geological, hydrological, and geophysical studies, and measures the water levels in water tables beneath Paris to asses the risk of ground subsidence and to reduce the flood risk. Monitoring the galleries is one of the IGC’s key roles. The department also plays a role in providing information and regulatory guidance for land development and construction projects in officially designated high-risk zones. It is also in charge of ground consolidation work for the City of Paris. More than sixty people work for the Department of Mining and Underground Works, which is split into four divisions: administration, preventive procedures, ground studies and ground consolidation, as well as the inspection, mapping, research, and studies division where Jules Querleux works. This division ensures that all mapping is accurate, participates in risk assessment studies, and manages all gallery monitoring and maintenance work. His team includes 5 engineers, 3 cartographers, 10 field agents, 1 brigade chief, and 1 heritage specialist.

Hundreds of kilometers of tunnels

Jules Querleux explains, “Every year we cover hundreds of kilometers of accessible tunnels to visit beneath Paris. There are potentially more. These are former limestone quarries from the Lutetian period. There is a high concentration of tunnels in the southern end of the capital, in the 5th, 6th, 12th, 13th, 14th, 15th, and 16th arrondissements. The northern districts of Paris also contain a latticework of underground tunnels where gypsum was mined to make plaster. Extremely fragile, these quarries are not at all accessible.”

Working in the caves of Paris

There are always at least three people working underground. Maintenance work, depending on the extent or nature, is handled either by the department itself or by specialized companies. One of their key roles is to monitor and assess the level of risk. “We closely monitor groundwater tables and the risk of collapse, which allows us to make a subterranean hazard map. The galleries and tunnels are, on average, 15-20 meters below the surface. Access from the street is either via ladder or hoist. Everyone is equipped with the proper clothing, boots, gloves, a helmet, lighting gear, waders for flooded areas, and a harness for lowering.”

Today, access to the tunnels is done either by using a hoist or a fall-arrest system. However, Jules Querleux, a caver in his spare time, thinks that in certain situations (such as when it is too difficult to place a hoist) learning how to use a rope and the requisite gear could be a workable solution for these “Parisian caves.”

Currently the IGC conducts from 5 to 10 interventions per week. The department provides an essential service to the city. “We receive a lot of calls and requests, and will be even busier with the current construction underway for the Greater Paris Area; these projects will considerably impact the city’s network of underground quarries. Just imagine when the huge tunnel-boring machines pass beneath the highly-populated areas of Paris.” Jules Querleux and his team are nowhere close to being finished surveying the depths of Lutetia!

*The well known Catacomb Museum in Paris is part of this network of underground tunnels and quarries.