Minecraft wiki entry for Mob_farm, associated text: Horizontal Transportation can be done easily using flowing water, with a drop of 1 block vertically for every 8 block traveled horizontally. Build a tunnel with a height of 3 blocks and a width equal to the size of your gathering holes (usually 2). Mine forward to a length of 8 Blocks, so that water placed at the start ends exactly at the drop. Repeat, but one block lower, so that the mobs fall from one funnel part to the next. Mine out the roof at the higher level for 2 or 3 more blocks to prevent spider clinging to the walls from clogging up the funnel.