Minecraft wiki entry for Shelters, associated text: First, dig a 2 block deep hole in the ground. Dig another hole 3 blocks deep, 1 block away from the first hole. Dig out the block that separates the bottom of the 2-deep hole with the middle of the 3-deep. If done correctly, this should allow all mobs (except for spiders) to fall into the 2 block hole. Then, you simply go into the 3 block hole and attack their feet. Murder Holes often function best alongside a perimeter wall of some sort, with the 2 deep outside and the 3 deep inside. An extra addition to this is to make a large amount of murder holes, and form a tunnel system of sorts, so you can watch multiple holes at once. These were featured by Paulsoaresjr in his Minecraft tutorial videos. This works best before version 1.2 because the mobs have new coding that makes it hard to get them to fall in holes. They will simply navigate around holes deeper than 2 blocks. A solution to that is to put signs around the holes but is usually more effort than worth.