So I was channeling out parts of the hillside into which I had dug, right next to my trade depot and outdoor meeting area. Suddenly a miner somehow caused a cave in -- which is a little suspicious because there was only one level being channeled at the time, but who knows, I may well have messed up there.

But this cave-in was very strange -- combat reports show traders, miners and nearby geese hit by white sand and boiling magma! The rock layer is obsidian and the soil nearby is loam, there's no sand on this map and I have not yet struck magma. Looking closely shows that a tile under the deconstructed-by-impact trade depot has changed to white sand. I think I remember a bug like that years ago, so perhaps that's not too unusual.

But magma? That's crazy. I can't imagine how a channeling mishap on the surface really shouldn't spawn magma! I seriously doubt that the depot deconstructed with sufficient force to melt nearby rocks.