When adventuring, there are frequent cave ins - sometimes only a few Z Levels, other times it is very many Z levels resulting in instant death.

World generation was pretty standard, only increased volcanism as far as I can remember.

The cave ins happen in multiple biomes, so don't appear to be linked to the Tundra that my adventurer is currently located.
Only mod active is DFHack, but this wasn't active during worldgen - I added this to try and teleport myself out of the fist big cave in.

Details about the world - there is only one player fortress, Littlebit of Cats in the far south east, capital / mountainhome of the Misty Bells. Circus breached and clowns defeated via oscilating spikes.

I can supply a map with several of the cave ins I've found if helpful, but they seem to be all over the southern tundra area I'm currently in.
There is one to the far east of the current location, where the tundra intercepts a river, and one in the bright blue ice area to the south East.