Minecraft wiki entry for Structure, associated text: Sponge rooms, 1×1×1 basic unit, have openings only to the bottom. Each of the 36 columns in the room has a 1⁄3 chance of being empty, a 1⁄2 chance of having one sponge, and a 1⁄6 chance of having two sponges. The walls have dark prismarine.