Ocean Monument is a large, rare underwater structure that generates exclusively in deep ocean biomes and their variants. It stands as the only natural source of sponges in the game and one of just two structures where prismarine can be found naturally — the other being ocean ruins. Unlike many other generated structures, the Ocean Monument contains no loot chests anywhere within it. Players looking to locate one can obtain an ocean explorer map by trading with a journeyman-level cartographer villager, which marks the nearest monument on the map.
The entire Ocean Monument is constructed from prismarine, prismarine bricks, dark prismarine, and sea lanterns, with a base footprint measuring 58×58 blocks. Large pillars extend downward from the structure toward the ocean floor. Inside, the central section features a series of randomly arranged chambers connecting to two lateral wings, while at its core sits a tall treasure chamber containing eight gold blocks encased within dark prismarine. Some monuments also feature sponge rooms, each holding wet sponges along the ceiling, though these rooms are not guaranteed to appear in every Ocean Monument.
Three elder guardians always inhabit the structure: one positioned atop the central section and one within each side wing. These formidable mobs apply Mining Fatigue III to nearby players, significantly slowing any effort to break through the monument's walls. The surrounding area also serves as the exclusive natural spawn location for ordinary guardians, which continue to appear there indefinitely. No more than one Ocean Monument can generate within any given 512×512 block region of the world.