Minecraft wiki entry for Mob_grinder, associated text: To make the grinder, first dig a 1×1 pit at least two blocks deep so mobs can't jump out. As mobs fall into the hole, they accumulate to the point where they suffocate to death. Collect the items with hoppers or hopper minecarts, as with any other grinder. When the farm is turned off and the density of mobs in the hole isn't high enough, they remain stuck in the chamber, but they can be finished off with a sword.