The Yonaguni Monument is located in the Ryuku Islands, which are part of Japan. They are also sometimes referred to as the Yonaguni Submarine Ruins. The monument is made up of rocks that sunk from the coast of the Yonaguni island. The monument has become a popular diving spot because it is home to many hammerhead sharks. Historians believe that the sandstone and mud rocks which make up the ruins are over twenty million years old.