The Grauniad: [S]ince 1955, for security reasons, it has been an offence to “penetrate into or circulate within” the rest of [Les Catacombes]
There exist, however, several secretive bands of so-called cataphiles, who gain access to the tunnels mainly after dark, through drains and ventilation shafts, and hold what in the popular imagination have become drunken orgies but are, by all accounts, innocent underground picnics.
Hat tip to Hanna.