Minecraft wiki entry for Terrain_features, associated text: Since water generates on the surface of the world below y=63, land it covers can turn into large lakes. These lakes are often quite shallow, and are never more than a few blocks deep. Large lakes generate pockets of sand, gravel, and clay on the lake floor, also known as disks. When large lakes generate near the shore they can turn into small bays. Unlike rivers, there is no seagrass on the lake bed.