The future residence of Blooz's Home for Wayward Mobs (BHWM). Though you can't exactly see it from this angle (I wanted to get a show of the depth) This hole starts at sea level. That is a boat floating on the water in the upper right. Also, to give you a sense of where I'm digging the wall on the left is the edge of the world. More about this below.
And here we have a view from the top of the completed dig. The black paths you see at the very bottom of the dig are bedrock. I forgot to check at what y my sea level begins. I do know the first landing there on the right is about y=40. You can also see, in center frame, the beginning second tier of the BHWM. I'm doing a modified verison of Ethos's practical mob grinder from the youtube video. As you can see, I've tried to address the flaw Ethos noted in that I've expanded the dump hole from 1 block to 4, meaning mobs shouldn't stack up as much. Of course, doing so forces the pin-wheel effect you see.
Here we are from bedrock, looking back up. Again, at this point of this picture, the BHWM only has one tier. Given the depth I have available, I should be able to build 6 tiers without ever going above the roof. (And yes, the roof is underwater, so the whole thing is invisible on a map .
So why build where I did (in the the very corner of my world). The biggest determent to a mob grinder's efficiency is unlit caves beneath it. I reasoned that, by building in the corner of the world, underground, I would minimize this impact. While digging this hole, I only found one cave system - you can see the steps up to it in the picture above. - and that only extended for about 100 blocks. I did find a dungeon embedded in the world's edge. That was an intersting find in that, because it was in the world's edge it was only about half the size of a normal dungeon. Still trying to figure out what to do with it.....