I built a tall 3x3 open space underground. I opened the top center tile so that the center column, and only the center column, was "Above Ground". I build a 3x3 retractable bridge across one of the levels (a few down from the top) and linked it to a lever. I put 3/7 water on the bridge.
Winter came. A single block of ice appeared, in the same tile as the bridge center. Then I retracted the bridge. Then I deconstructed the bridge, just to make absolutely sure.
The block of ice is still there, floating in midair.
For my next trick, I'm going to try to build a bridge to the top of the ice block and build a stone floor on top of it. Meanwhile, I've backed up my save. Engineering using floating ice could have...
interesting applications. It could potentially even have Fun applications.
EDIT: Wow. The ice fell as the instant I
allocated the creation of the bridge to it. That was disappointing at first, but now it seems even more awesome, because it could give a player
instant control of deathtraps. Of course, that would be an exploit, but it's only fair when the game is unfairly stopping me from creating automatic cave-ins (which is what I was originally trying to do with this ice trick.)
EDIT: I also built a wall on top of some ice. The block remained in midair after the ice melted under it. It didn't fall until I
completed a bridge touching it. Trapping enemies on floating blocks by
dropping them there would probably work, and would be pretty cute (albeit useless.)