http://i.imgur.com/1pVuS.png This is the underground blueprints, under the cube. It's very important to put torches in all caverns under the cube if you want to have good spawning. Every block you remove under (or above) your spawning room also helps the spawning. Investigate the area whenever you hear monsters or water flowing. If you want to make a diamond mine or get cobblestones, the best place is right under it.