Inspired by this topic i have decided to build a similar creation. The test setup will be built as following: A 3x3 room above me entraceway is channeled out and bridged over with a retracting bridge. Water will be pumped up from the cave river from one direction. A floodgate is installed to prevent flooding the tunnel upon use. A drain is installed on the other side, also with a floodgate. This will further reduce the mess downstairs. A 1 wide tunnel is dug over to the middle of the preperation chamber, channeled and hatched over and locked with a floodgate, leaving a single tile with a hatch on the floor. The higher tunnel is filled with magma, the internal gate opens, letting magma entr the dropping room and is closed again. Then the hatch is opened, dropping the tile of magma into the pool, obsidianizing the center tile.
Now there's 3 things that can happen:
1. The obsidian teleports through the bridge into the tunnel and imediatly causes a cavein there.
2. The obsidian smashes through the bridge and causes a cavein along with the bridge materials.
3. The obsidian rests upon the bridge until it's retracted after which it collapses.
One is by far the best case scenario. The drain feature will become unnesacery(how do i configure firefox spellchecker to help with english instead of dutch?) and the trap can be operated by staggered use of the magma channel openings.
The second is the worst but considering the power of the trap, an acceptable ammount of work. There will be a considerable "afterbirth" of water as AlienChickenPie put it so politely and to reload the trap, the bridge must be rebuilt and both rooms flooded again.
The third makes maintenance alot easier and it allows for the trap to be preloaded long before invaders arrive. Drop magma, close magma hatch, reload magma chamber, drain excess water and wait. The trap can also be snapfired without draining water is case of an emergency and can be reconfigured for both water and magma flooding. Not reccomended for encasing purposes because of the risk of the tube clogging up.
The trap can be resized at will as long as the lower room is always one tile larger in all directions as the one above. However, load times increase exponentiay unless you use a vertical fluid storage, which will give you multiple uses before the whole thing needs to be recharged. Non-issue with theory 2.
Crude paint sketch: