Minecraft wiki entry for Underwater_home, associated text: The traditional method is to fill the space with a falling block (sand or gravel) and then dig it all back out. This method is well suited when you have constructed all of the walls and then cap them with solid blocks once the space is filled in. If you have slime blocks, honey blocks or TNT available in quantity they make a good filler as they can be insta-mined using just a fist and so can be broken even when you are fatigued by an elder guardian.