|I found a nice embark site where my dwarves could start ON a frozen river that didn't thaw until mid spring. That gave me (after a few tries and some experimenting and restarting repeatedly) enough time to dig through and wall around, and so forth - quite standard tactic, really. Anyway, there was also a waterfall that, in my previous attempts, had caused the river I was in to overflow when my fort impeded the flow, so I had walled off not only the underwater area, but put a nice layer of walls on the surface level of the water as well. At the time when I did this, the ground was merely the surface of the frozen water, entirely ice. I proceeded to dig down and transport everything just fine into my secret river hideout.
When spring came, I found out that this time around I had not blocked flow enough to cause overflow, so I decided to remove that layer of walls and build all floors around it (I can't dig around because all the stone is aquifered conglomerate) When I removed the walls, however, even though all the ice had melted, the ground where the walls had been was still ice, and it has not yet unthawed even though I've let it sit for a while.
Basically, when a wall is removed it leaves ground of whatever the soil type in that area is. When I originally placed the wall, the soil type was 'ice'. Now that I removed the wall, the ground reverts to its original type when the wall was built, which is ice. This was in the late spring, and in said for it is now early fall and the ice remains yet.