Minecraft wiki entry for Shelters, associated text: If you are in multiplayer, you don't want anyone getting into an underground base, right? Well, dig down at least three blocks and make a room. Then get out and put a piece of dirt over the entrance and place a sapling. Put bonemeal on it. Then dig the piece of dirt out and make a hill one block away from the tree. Dig one block into the hill and place sticky pistons so that when they come out they stick to the tree trunks and pull it back or forth, thus opening the entrance and closing it. After that, you need to expand the base so it's actually useful!