There was a relatively small (9km crater) marine impact (55+/- Ma)--Ragozinka--in what are now the Middle Urals in Russia, which could have easily triggered a methane release.
Cool!
I'm also a fan of the idea of bolide impacts triggering volcanism elsewhere
on the globe, especially in the vicinity of their antipodes (e.g. Chicxulub
and the Deccan Traps).
BTW...
Also as the the vapor condenses it extracts heat from the atmosphere.