A geological deception
Off the southern coast of Japan's Yonaguni Island lies a massive underwater rock formation that appears deceptively artificial. Discovered by chance in 1986 by diver Kihachiro Aratake, the primary structure measures roughly 100 meters long by 60 meters wide and rises 25 meters from the seabed. Its most notable features are the immense, parallel terraces with seemingly flat faces and sharp, 90-degree angles that make it look like a submerged, stepped pyramid.
The visual evidence is compelling. Divers can swim along features nicknamed the "loop road" and observe structures that resemble walls and columns. One section looks like a giant turtle carved from the rock, while another triangular depression is called the "triangle pool." These features have led some, including marine geologist Masaaki Kimura, to propose that the monument is a 5,000-year-old city, sunk by tectonic activity. This interpretation is not supported by the geological evidence. Neither the Japanese Agency for Cultural Affairs nor the Okinawa Prefectural government recognizes the site as a cultural artifact.
The case for nature
The scientific consensus is that the Yonaguni Monument is an entirely natural formation. Geologists, such as Robert Schoch of Boston University, point to the fundamental properties of the rock itself. The structure is composed of medium-to-fine sandstones and mudstones from the Early Miocene Yaeyama Group, deposited about 20 million years ago. This type of sedimentary rock has well-defined, parallel bedding planes where layers easily separate.
Yonaguni Island is in a region with high seismic activity, and the rock is crisscrossed by numerous sets of vertical, parallel joints caused by tectonic stress. The combination of horizontal bedding planes and vertical joints means the rock naturally fractures into rectangular, block-like shapes. The powerful Kuroshio Current, along with wave action over thousands of years when sea levels were lower, eroded the softer rock along these fracture lines. This process left behind the harder, cleaner edges that look like cut stone steps. The entire structure is carved from solid, underlying bedrock, not assembled from separate blocks, which would be expected in a true construction. Similar erosion patterns and sharp-angled fractures are visible in the sandstone cliffs on Yonaguni Island itself, above the water.