Mines of Paris
Exploring the mines is not allowed by the prefecture and people caught get heavy fines.  A limited part of the network (1.7 km) was used as an underground ossuary (storage place for human bones) in the past known as the catacombs of Paris, and can be legally visited from the entrance on Place Denfert-Rochereau. The entire network is commonly but mistakenly called "the catacombs". Despite restrictions, the network is often toured by urban explorers popularly called cataphiles.