There is a formation of man-made monoliths that lie submerged deep underwater off the coast of Ryukyu Islands, Japan’s southernmost islands. Its actual name is Yonaguni Monument. There is a debate on whether or not the monoliths are completely man-made or if it was a natural site that has been modified. Neither the Japanese Agency for Cultural Affair nor the government of Okinawa Prefecture recognize this as an important cultural artifact or have studied it at all.
These monoliths were first discovered in 1986 during the winter months, this is because the sea around Yonaguni is a popular diving spot because of a high number of hammerhead sharks are seen there. Lately, the formation has been a huge tourist attraction. It has been informally studied to some extent but to no avail.
The monument is composed of medium to fine sandstones and mudstones of the Lower Miocene Group believed to have been deposited about 20 million years ago. Most of the formation though is connected underneath to rock mass. The monument itself is about 150 by 40m and 27 m tall.