Built in the late 18th century, these subterranean tunnels now house the remains of over 6 million people. Mass graveyards were overflowing in Paris, so bodies were exhumed and transported there. During WWII, the Resistance used portions of the Catacombs as hideouts, even Nazis built bunkers down there.
There are tour guides given for the portion of the catacombs that are open to the public, though there are numerous secret passages and entrances throughout. If you take a tour guide, one of the first signs you will see reads “Arrête, c’est ici l’empire de la mort” which means Stop, this is the empire of death.
Though only about a mile of the catacombs is open to the public, it doesn’t stop people from sneaking in. People are advised against that as even experts get lost because a lot of the tunnels are not mapped. There is a legend of a man named Philibert Aspairt who got lost in the catacombs in 1793. His body was found 11 years after his death and its said that it was near an exit.