Minecraft wiki entry for Exploring_caves, associated text: Caves or caverns are naturally-formed cavities in the Overworld and less prominently, the Nether. Most caves have tunnels branching off as catacombs and winding in all sorts of directions. They are often very deep and can go from the surface all the way to bedrock. Caves are not ravines but will often lead into them. Because they mostly lack illumination, they will often have groups of hostile mobs like skeletons, zombies, spiders and etc.