Mines of Paris

The Mines of Paris (in French Carrières de Paris — "quarries of Paris") are made up of a number of abandoned, underground mines under Paris, France, connected together by large chambers called galleries. Three main networks exist; the largest, called the "grand réseau sud" ("large south network"), lies under the Ve, VIe, XIVe and XVe arrondissements, a second under the XIIIe arrondissement, and a third under the XVIe, though other minor networks are found under the XIIe, XIVe and XVIe for instance. Together, the mines and galleries run for about 280 kilometres.
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.
Ossuary.
During the 18th century, the growing population of Paris resulted in the filling up of existing cemeteries, raising public health concerns. Towards the end of the 18th century, it was decided to create three new large cemeteries and to condemn the existing cemeteries within the city limits. Human remains were slowly moved to a rebuilt section of the abandoned mines that would later become a ossuary. The entrance is located on present day "Place Denfert-Rochereau".
The ossuary became a tourist attraction on from the early 19th century and has been open to the public on a regular basis from 1867. Although it is officially called the "Ossuaire Municipal", it is widely known as "the catacombs". Though the entire network of Paris' mines is not a burial place as such, the term 'Catacombs' is commonly used to refer to the whole network.