Minecraft wiki entry for Ocean_Monument, associated text: Ocean monuments generate in areas with deep ocean biome variants everywhere within a 16 block square radius around the center point, and an aquatic biome (regular/deep/frozen ocean, regular/frozen river) everywhere within a 29 block square radius around the center point. In order to prevent air pockets, water generates around the monument as a part of the structure. It is the largest structure that does not contain a loot chest and the largest water structure.