I was making a fun video when I got this idea. I want a way to get my mob farm to flood the area above the grinder with monsters for an epic battle. I tried a regular waterflow from top to bottom and this creates a strong downward pull. You swim up it very slowly and mobs might drown or stay stuck to the bottom.
Then I got this idea. You line the walls of your elevator with signs or ladders every other layer, and place water source blocks inbetween. This lets you swim straight up without any downforce affecting you or mobs, and there is an airpocket every other layer, so you can't drown, and more importantly, the mobs can't drown.
This means you can put your grinder anywhere in relation to your mob spawning tower. It used to be limited from gravity, mobs only flowed downhill. The only mob that doesn't go up the advanced elevator are spiders, they just sit at the bottom. I think because they are just 1 block high, they can't jump from one layer up to the next.
I have a video uploading now with a demonstration and an epic battle. At the end, just before my 10 minute limit, I peak inside the mob tower to show off the elevator.