In 1987, during a dive led by Japanese Kihachiro Aratake off the southern coast of Yonaguni, which is the westernmost island of Japan, a huge stone structure, later known as the "Yonaguni Monument", was discovered on the seabed.

In 1987, during a dive led by Japanese Kihachiro Aratake off the southern coast of Yonaguni, which is the westernmost island of Japan, a huge stone structure, later known as the "Yonaguni Monument", was discovered on the seabed. At first glance, it appears to be in front of an architectural work, part of which is sandwiched between two pillars rising 8 metres high. Since its discovery, many scientists have begun to study this phenomenon despite the presence of strong ocean currents that still make diving at this site very difficult. However, over the years, traces of flora, fauna and stalactites have been found, which usually only form on the surface. Here it seems that the sea bottom contains the remains of an ancient civilisation. The similarity to other finds from the ancient world has led many to consider this structure, 120 metres long, 40 wide and 20 to 25 high, as an artificial monument from 10,000 years ago. Other analyses indicate that the origin of the structure may date back to around 8,000 years ago. During the exploration of the site, structures similar to cobbled streets, squares, intersections and stairs were identified. The outer walls in some cases are smooth, with many right angles at 90° and well defined, making the structure look like a kind of terrace. Some geologists consider the Yonaguni monument to be a simple natural formation.