The Yonaguni Monument, located off Japan's Yonaguni Island, is a geological formation situated approximately 6 meters below sea level, extending to a depth of 24 meters. Discovered in 1987 by diving instructor Kihachiro Aratake, its appearance, characterized by stepped sandstone and mudstone with squared edges, has led to comparisons with ancient structures.
A debate exists regarding the monument's origin. Geologist Masaaki Kimura has suggested the structure was modified or constructed by human hands before being submerged around 10,000 years ago, though this view is controversial within the geological community.
Most geologists assert that the Yonaguni Monument is a natural formation. Its structured appearance is attributed to natural processes, including tectonic stresses and erosion acting along fractures and bedding planes over thousands of years.
Relevant geological features include bedding planes, which are natural layers in sedimentary rocks, and joint sets, which are parallel fractures that can split rock under stress. Geologist Robert Schoch noted in 1997 that Yonaguni is an earthquake-prone region where seismic activity can regularly fracture rocks. This activity, combined with erosion from ocean currents, is proposed to explain the monument's stepped and geometric forms.
Other examples of naturally geometric rock formations include Ireland's Giant's Causeway, Scotland's Fingal's Cave, Tasmania's Tessellated Pavement, Saudi Arabia's Al Naslaa rock, and Norway's Preikestolen.
A 2024 study by a team of geologists led by Hironobu Suga of Kyushu University reported no archaeological remains or traces of human activity at the site. Their underwater observations documented ongoing erosion processes, such as bedrock detachment, abrasion, and the formation of potholes, supporting the conclusion that the monument's formations are a result of continuous natural weathering and erosion of sandstone on the seafloor.