Paris has the Catacombs. A very small part is opened to the public as a museum, but most of the area is the territory of underground explorers. https://en.wiki

Paris has the Catacombs. A very small part is opened to the public as a museum, but most of the area is the territory of underground explorers. https://en.wikipedia.org/wiki/Catacombs_of_Paris https://en.wikipedia.org/wiki/Catacombs_of_Paris