Sometimes you want to funel mobs coming from all for directions into bubble columns. This design pulls mobs into two bubble columns inside a 2x2 area. The glass panes can be replaced by solid blocks.

There are layer-by-layer images included in the media gallery. The slabs need to be waterlogged. To use glass panes, replace the yellow, lime, light blue and blue wool with panes and replace the pink wool with daylight detectors. The latter is necessary to allow mobs to drop into the waterstream, while keeping the streams from joining each other.

The map is working in Minecraft 1.13 and 1.14-prerelease-4

The 3D-preview seems to be broken for my map.