Minecraft wiki entry for Underwater_home, associated text: Find a fairly deep patch of water with a relatively flat floor. Then build a platform on or above the water surface out of any non-gravity block that is easy to remove such as dirt, wool, or leaves. Using a cavity block, like gravel or sand, build the shape of your desired building on the platform like building a "sand castle", but without the exterior walls. You won’t be able to have any overhanging exterior features and the interior must be a solid mass. When the "mould" is complete, remove the supporting platform blocks to let the gravity blocks sink to the bottom.