Apparently there is a group of people who like to descend into the catacombs that weave their way through the Parisian nether world to party the night away. This, of course, is frowned upon by the local authorities who engage in a game of cat and mouse with the revelers.
This is not new, as groups before used the catacombs to meet secretly; in a far more deadly game. The French resistance used thier knowledge of the catacombs to send messages and hold meetings during World War II.
I visited the catacombs once; it was both intellectually interesting and a bit scary. A lot of souls rest there.