I'm trying to build an undersea fortress. Dropping an obsidian bathysphere didn't work (kills the dwarves inside). Draining into an aquifer? 5 dead miners later I've decided it just won't work. That leaves: lignite bin, or drop cast obsidian in the ocean.
I don't feel like messing with the lignite bin, so that leaves making obsidian walls. But what's always bothered me about that is how do you get rid of the walls afterwards? I mean, I want the walls 100% gone when I'm done. Not just water within the walls, but unobstructed view of the whole ocean. Since they're cast walls, not constructed, the whole wall is not going to disintegrate just because it falls 1 Z level.
I think I've figured out how to completely remove a cast wall in the ocean when you don't want it anymore. When you're ready to get rid of it, you dig a cavern under it down many Z levels, as deep underneath the ocean floor as the wall is tall. You dig the cavern 1 tile further out than the wall is all the way around. Channel out 1 tile around the inside of the wall too. Then you cast another ring like you did to build the wall, only this ring will be 1 tile farther around on all sides. So when you drop it, it punctures the floor on the outside of the wall, and falls through all the Z levels puncturing floors until it hits bottom. Then, since you dug out as many Z levels down as the wall was tall, it falls that many Z levels down and leaves you with a clear view of the ocean.