[adrianN]

In a place with sufficiently tight exits like the crawl holes you mentioned, it is enough if there is a little smoke to cause a panic. People will die from being trampled even if there is no actual danger from the fire.

[kehrlann]

Home-made smoke bombs are a classic (and annoying) prank in the underground. Fills an entire gallery in less than a minute: https://www.loupiote.com/photos_l/catacombes-de-paris-cataco...

Parties I was talking about look like this: https://youtu.be/6E-F-cVqubU