Sometimes there are parties with tens or over a hundred people in a big room that only has two crawl holes to exit it.
About fire safety, well it's pretty damp down there, mostly with rocks and dirt. Can't picture a whole room catching fire... But I'm no expert in fire safety.
In my uninformed opinion, this seems unlikely: gas pipes in Paris are buried less than 1m deep, whereas the catacombs are 10 to 30m underground, below service galleries for utilities, sewers, and even the subway.
Parties I was talking about look like this: https://youtu.be/6E-F-cVqubU
The surroundings were almost steampunk, with old pipes and boilers going each very way. Or maybe a WW2 submarine, but larger.
Anyway, I remember an /extremely/ /interesting/ chat (as one is want to have under the influence) with someone working there, in which he assured me they had passed all fire inspections. I had inquired because the only ways out were two extremely steep staircases, barely fitting two people side-by-side each.
Came back a week later only to find it closed. Turns out they never had any permits for the club, only for art exhibits on the ground level. It became a minor political scandal. IIRC there were two authorities who each assumed the other had jurisdiction.
This was underground by location only. It was among the top ten on residentadvisor, in travel guides, etc. Maybe a few hundred people on a Saturday night. I was there with a friend who did press for the mayor. Sort of hiding in plain sight.
FWIW, this was sort of a singular incident as far as I can tell. All the clubs in this town are repurposed, but I haven’t seen any other death traps, and I do tend to try the exits. It also helps that pyrotechnics aren’t big in the scene here, and that LED lighting has taken over.