Minecraft wiki entry for Shelters, associated text: As with "Hole in the wall", you can dig into the cave's wall while waiting for dawn, but keep some blocks handy to patch up any openings you might make into another cave while digging, which might have a hostile mob in it. If you place your pickaxe one of your hotbar slots (used to navigate quickly between items you are carrying in your inventory) and e.g. dirt is placed right next to it, you can quickly switch between your tool and that block type in order to close the hole you just made. This can be extremely handy when you stumble upon an opening which contains mobs.