Yonaguni Monument

Submerged sandstone formation about 100 m off Arakawabana Cape on the southern coast of Yonaguni Island, discovered in 1986 by local dive operator Kihachiro Aratake while scouting hammerhead habitat. The structure features terraced platforms, broad flat surfaces, sharp right angles and what divers call staircases, spanning roughly 50 m across and rising about 25 m off the bottom. Geologists generally read the geometry as erosion along bedding planes and linear joints in soft sandstone; a minority argue for prehistoric human modification. The main monument tops out near 5 m and its outer face drops to 25–30 m. From January through March schools of scalloped hammerheads pass the same southwest coast on rising current — sometimes more than 100 sharks in a single sighting.