Minecraft wiki entry for Time-saving_tips, associated text: When mining a new area, place barriers at intervals. Fences and cobblestone walls are ideal. This helps stop mobs sneaking up behind you unexpectedly, and allows you to mark sections of your mine as 'safe'. If you encounter a mob in a 'safe' area, it will probably have been stopped short by your fence, preventing it from attacking you, and you now have an indication that an area is insufficiently lit, or that there is an opening you have not discovered. This trick can save a great deal of time in narrowing down where an unexpected mob has come from.