Minecraft wiki entry for Construction, associated text: This is a type of building under the ground, like a bombing shelter or nuke shelter. This is for a more hidden type of shelter under the ground/ocean and makes a nonpublic safe house. They may be hard to find, so create a landmark to let you know it's there. It is safer than a shelter above ground. Advantages are no one/thing can find you, and you are more protected. You can also use it as a point where you can have a mine. Disadvantages are you can't have windows, so you have to actually exit your shelter to see outside (or make a hole in the ground to see outside) and the fact that every expansion requires a pickaxe. Also, if you break a hole and there's lava or water, you are stuck and you are very likely to die.