Entrance to the catacombs is restricted. The portion of the catacombs open to the public is only a small part of an extensive network of underground tunnels, which spans more than 300km (about 186 miles) in length. The tunnel system is complex, and though some tunnels have plaques indicating the name of the street above, it is easy to get lost. Some passages are low or narrow, and others are partially flooded. There are aging telephone wires, pipes, and such that can hinder progress, and cave-ins, although rare, do occasionally occur. A good guide is indispensable, and many guides occasionally refer to a map. Because of these dangers, accessing the catacombs without official escort has been illegal since November 2, 1955. There is a € 60 fine for persons caught by the cataflics — the special police who patrol the catacombs.
Secret entrances exist throughout Paris, and it is possible to enter the catacombs via the sewers, metro, and certain manholes. Some unofficial visitors hold keys to certain official entrances. On rare occasions people use these access points and illegally enter the catacombs — for example, to meet clandestinely, to hold unusual parties, or simply as urban explorers. (Those with an affinity for exploring the catacombs are known as cataphiles.)
In September 2004, an underground movie theater run by the Mexican Perforation — a French artistic movement that seeks to convey their ideas using underground places — was discovered by the French police.
Cataphiles often descend for a day, a night, or perhaps a week to explore, photograph, paint murals, create maps, clean up rooms, dig chatières and enjoy themselves among friends.
Cataphile ethics include leaving no waste behind, and never leaving manholes open (which would create a danger for the street-side populace, and inspire officials to seal the entrance). Tagging, a form of graffiti consisting of a stylized signature, is a controversial subject for cataphiles, and urban explorers in general. Some cataphiles tag, but others view the practice with disdain.
Some cataphiles leave tracts -- pieces of paper featuring drawings, poetry, photos, jokes, or a historical explanation of a certain underground site. Often tracts are hidden in nooks and crannies. Many tracts are detailed works of art, and many cataphiles collect them.