Yonaguni Monument
A large submerged sandstone formation off Yonaguni Island, Japan, with strikingly flat terraces and right angles that some interpret as a sunken man-made monument and most geologists regard as natural.
The Yonaguni Monument is an underwater rock formation discovered in 1986 off the southern shore of Yonaguni Island, the westernmost of the Ryukyu Islands. It sits about 25 m below the surface and measures roughly 150 m long, featuring stepped terraces, flat planes, and apparent right angles in its sandstone bedding. Because the area was above sea level during the last Ice Age, an artificial origin would push the structure’s age past 10,000 years. The genuine dispute is whether the formation is natural geology or shows human modification.
Marine geologist Masaaki Kimura argues the layout is too regular to be natural, reporting what he interprets as a road, terraces, and carved features. On Ancient Aliens and within the ancient-astronaut hypothesis, Yonaguni is folded into lost-continent lore (Mu) and cited as proof of a vanished advanced civilization predating recorded history.
Most geologists, led by Robert Schoch, conclude the monument is a natural formation. The local fine-grained sandstone fractures along straight, parallel bedding planes, producing flat surfaces and step-like edges, and the site lies in a seismically and hydrodynamically active zone where jointing and current scouring readily create rectangular shapes. No unambiguous tool marks, transported stones, or artifacts have been recovered.