I'm a lazy bastard so every fort I've started so far I've started on the rim of a volcano for easy magma access. In this one particular fort, though, I noticed something weird when I went to check where the volcano was to see where the best place for my fort entrance would be.
In two areas around the crater, I noticed... puddles. Just couple thin, 1/7 water tiles. That's weird, I thought, and then didn't really give it much more thought.
I'm in a temperate zone, though, so come winter, all the water on my map froze. Dum de dum de dum, kept playing, setting up my fort, la la la. Then spring comes, and suddenly I get the notification, "A section of the cavern has collapsed!"
I had just been channeling out a eventual magma moat, so I was like, "Damn! Did I mess up!"
Then to find out that what collapsed was something inside the volcano.
Curious, I go to look and find that the puddles of water have become rather significant torrents which are flooding into the volcanic vent from both sides. Weirded out by this sudden influx of sourceless water, I check around to discover that apparently on opposite sides of the volcano there are two areas where the floor is labeled "murky pool", but which unlike normal murky pools do not appear to have walls, but instead abut directly on the volcanic crater. Thus whenever water falls into them, it immediately flows out in to the volcano.
It seems freezing "increased" the volume, since presumably the game doesn't track whether the water that froze was at 1/7 depth or 7/7 depth, and so when it unfroze a sudden torrent of water occur.
Still, I'm wondering how rare this is to have murky pulls inside a volcanic crater. I'm also wondering if I can put it to any nefarious purpose...