China Miéville’s fantasy novel Un Lun Dun hints at the existence of twisted sister cities, and Paris is no exception. Below the City of Light lies the Paris Underground, a vast network of tunnels and galleries, most of which are off-limits to the public. These tunnels were created through mining operations for limestone and gypsum, which were used for building materials in Paris. The catacombs, which house the bones of six million Parisians, are just a small part of this underground labyrinth. The tunnels have served various purposes throughout history, including brewing beer, hosting parties, and even providing a potential source of geothermal energy. The city has sought to block off and control access to the underground network, but cataphiles continue to explore and defy the authorities. The constant temperature of the tunnels makes them a potential resource for sustainable climate control in Paris.
https://www.atlasobscura.com/articles/strange-maps-paris-catacombs