I had a idea a while back, and sadly I was never able to spend the time trying it (job and family), but I am fairly sure it would work and be extremely deadly.
It's an automatic cave in machine
I'd like your feedback about it? Would it work or not, is it dwarfy ...
Essentially, the machine needs a 1z high corridor with natural floor underneath, a 2z level deep magma tank immediately above the corridor, and either a) a 1z level water tank with bridges as a base immediately above the magma or b)a 1z level water tank operated by a hatch hatch at least 2 z levels above the magma. This is how I imagine it would work:
The bridges would open and drop the water into the 2z level deep magma tank. The magma turns to obsidian on the top of the tank, and since a) there is no solid under the wall and b) bridge won't support the wall from above, the obsidian wall will fall.
It will fall through the magma at the bottom of the tank, smash the base of the magma tank, and stop in the corridor (since the corridor has a natural floor).
The magma tank should not leak, because the top of the obsidian wall (now in the corridor) should plug the hole that the wall made as it fell.
That is the cave in part.
To reload, have a miner dig out the obsidian wall in the corridor, and refill the tanks (magma + water).
I reckon the shape of the water tank could be adjusted to have different shapes of cave in.
Do you think this trap would work?