I embarked on a glacier with two layers of ice over rock. In order to get liquid water, I collapsed an 8x8 square of ice wall from the lower ice level down into a stone room beneath it, which then melted into water. :
----------- Ice Floor at ground level
| | Open ice room
- ----- - Ice Floor with channels
| |XXX| | Block to be collapsed
- ----- - Stone Floor with channels
| | Stone Room
----------- Stone Floor
|XXXXXXXXX| Solid Stone Layer
Some of the water was thrown back up by the collapse and formed partial ice blocks connected to the walls in the two ice layers above the stone room. I drained the water out of the stone room until the entire room was 1/7, and observed that bug #667 was occurring- that is, the floor of the room had disappeared even though there was solid rock underneath the water, so what I had was a 1/7 water layer in squares called "open space" even though there was solid rock in the layer underneath the stone room.
----------- Ice Floor at ground level
| |X| | Open ice room with partial ice block
- --- - Ice Floor with gaps
| |X| | Open ice room with partial ice block
- --- - Stone Floor with gaps
|~~~~~~~~~| Stone Room with 1/7 layer of water
--- --- Stone Floor with gap
|XXXXXXXXX| Solid Stone Layer
Hoping to get more water, I dug out the ice block in the upper ice layer and collapsed the ice block in the lower ice layer into the stone room. At this point, I was bombarded with "A section of the cavern has collapsed!", which made it to x 1243 with no sign of stopping. Turning cave-ins off in init resolved the issue; there were a couple of ice floors that were apparently collapsing into the stone room, which threw up 1/7 layers of water into the ice layer, which froze into ice floors, which collapsed into the stone room, ad infinitum. I believe that the collapse-throw water up-freeze-collapse cycle would probably have continued forever if I hadn't turned cave-ins off.