The Yonaguni Monument is a massive underwater rock formation off the coast of Yonaguni, the southernmost of the Ryukyu Islands, in Japan. There is a debate about whether the site is completely natural, is a natural site that has been modified, or is a human-made artifact. For these reasons, the site is also known in Japanese as the Yonaguni (Island) Submarine Ruins.
The flat parallel faces, sharp edges, and mostly right angles of the formation have led many people, including many of the underwater photographers and divers who have visited the site and some scholars, to the opinion that those features are human-made. These people include Gary and Cecilia Hagland and Tom Holden, who went on underwater expeditions to study and photograph the site. These features include a trench that has two internal 90° angles as well as the twin megaliths that appear to have been placed there. These megaliths have straight edges and square corners. However, sea currents have been known to move large rocks on a regular basis. Some of those who see the formations as being largely natural claim that they may have been modified by human hands. The semi-regular terraces of the Monument have been compared to other examples of megalithic architecture, such as the rock-hewn terraces seen at Sacsayhuaman. The formations have also been compared to the Okinawa Tomb, a rock-hewn structure of uncertain age.
