Minecraft wiki entry for Shelters, associated text: A big room with flat floor for growing mushrooms. You can light your farm by torches or glowstone, placed on ceiling, to prevent mobs from appearing. Just make sure that there are no places on the floor where the light level is greater than 12. If you have the material, Redstone torches are good for this. More about this can be found on the Tutorials/Mushroom farming page. Alternatively, find a small cave or make one, put some mushrooms inside and completely seal it off without torches and check on it/clean it out. Also provides the drops from hostile mobs.