Minecraft wiki entry for Building_a_metropolis, associated text: Mine More of a service than a building. Build ladders down to layer 12 and let people have easy access to ores. They can extend it and you can block off some caves and ravines. Destroy spawners in dungeons and operate a first come, first served policy. You can also use a deep, natural cave for a mine, if you find a good one. Remember, you cannot make any mines if you play on superflat, unless you customize it in certain ways. A good way to mine is to branch-mine. It's efficient, safe and useful. It is recommended in extreme hills because of the emeralds available. It might be a good idea to build this near a surface smelter or refinery.