Drowning Killing Mechanism
Drowning killing mechanisms work in two ways: Either forcing mobs to swim upawrds and get trapped, or forcing them down.
I described how to build one that forces the mobs down in the Mob spawner part of this section. So, i will explain how to create a way to force mobs upward.
To do this you use the same “dead end” idea as the lava killing mechanism, but instead of having floating lava you have floating water. You extend this a block or two higher as well. This has the effect that when the mobs get to the end of the passage they then swim up this floating water and get trapped, they will consequently run out of air and die. It is important that every block in this area is a source block, this is due to the fact that if the water is flowing down it will push the mobs down instead of letting them swim up.
Now, this area where the mobs swim up and drown can be extended as far as you like. If you put air pockets in sequencial steps you can in effect create a “mobivator”. This will transport your mobs to the ground level. You can then move them into another killing mechanism to then allow your items to easily be collected above ground.