Minecraft wiki entry for Shelters, associated text: If you found a cave system you can fix it up into a lair—a good one can make a base for the rest of your game. If it ends quickly, then cap off your entrance. If it does not end, then build a little shelter around you by capping off any extra exits or openings into the depths. Don't worry too much about the natural walls of the cave; monsters can still be heard through the walls pretty loudly, but they can't come through unless you leave an opening.