Minecraft wiki entry for Multiplayer_PvP_bases, associated text: Building the base: First, obtain some sponges. Either raid them from an ocean monument, buy them from the shop, or steal them from another person. Then, go find an ocean. A really big ocean. Then, go to the bottom and place down three sugar cane in a stack. You'll be able to breathe when standing in the sugar cane. Begin building your base above ground under the water. Don't worry about draining it yet, just build what you want it to be. Go ahead and build more space than you think you'll need so that you won't have to expand and flood your base again. Once you have it completely built and sealed from the outside water, take those sponges and use them to drain out the inside of your base. You now have an underwater base!