Minecraft wiki entry for Biome, associated text: These biomes are used for the generation of bodies of water such as rivers and oceans. Oceans are large, open biomes made entirely of water going up to y=63, with underwater relief on the sea floor, such as small mountains and plains, usually including gravel. Oceans typically extend under 3,000 blocks in any direction; around 60% of the Overworld's surface is covered in ocean. Small islands with infrequent vegetation can be found in oceans. Passive mobs sometimes can spawn on these islands, as hostiles can. Squid spawn frequently in the water. Underwater cave entrances can be found frequently at the bottom of the ocean.