The Yonaguni monument is a group of very big underwater rocks. It is off the southern coast of Japan. It was discovered by local divers in the 1980s. The monument has staircase-like terraces with flat sides and sharp corners. Most of academic society believe the rock formation is created by nature. Masaaki Kimura, a professor of earthquakes in the University of the Ryukyus, and some newspaper people believe it is a man-made structure engraved or built 2,000 to 3,000 years ago. The main feature (the "Monument" proper) is a rectangular formation measuring about 150 by 40 m (490 by 130 ft), and about 27 m (90 ft) tall. The top is about 5 m (16 ft) below sea level. Most of its top surface is a complex series of terraces and large wide steps, mostly rectangular, with walls that are almost vertical.