Minecraft wiki entry for Time-saving_tips, associated text: It's very easy to get temporarily lost underground, and to waste time wandering into areas of your mine that you have already cleared. By blocking off completed mine sections you avoid accidentally wandering into them again. Ideally, use glass, fence, or leaves. This allows you to see into the area, and spot any mobs that may appear (indicating either inadequate lighting or possibly a section you have overlooked). In particular, try only using materials not found naturally underground (cobblestone is close to ideal), as you may not be able to tell if the blocked area was blocked off by you, or if the apparent barrier is just naturally spawned. You can also use signs to mark off finished areas. Another good idea is to create a unique shape out of the blocks unlikely to generate naturally. In that case, you can use materials found underground.