Minecraft wiki entry for Shelters, associated text: Create a platform on top of a relatively flat surface or mountain. Dig out a trench in the middle of it and place redstone on either side. Connect the redstone to a single button and fill the trench with TNT. All you need to do is to press the button and you will begin a pit that leads to the bottom of the world. An automatic reloader can be added via sand and pistons. (This can be easily re-purposed into a cannon.)