[jacquesm]

Gas leak... depending on how far down you are, old gas lines tend to leak little bits of gas because they get porous that can pool.

[kehrlann]

Never heard of such an incident in the catacombs before.

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.