Massive underwater rock formations with right angles, flat terraces, and what appear to be steps. Geologists say natural fracturing can create such features. Others insist it's a 10,000-year-old structure submerged by rising seas.
Melkov, CC0, via Wikimedia Commons
A Submerged Discovery
In 1986, a local dive operator named Kihachiro Aratake was scouting for new hammerhead shark watching spots off the coast of Yonaguni, Japan's westernmost island. What he found instead, about 25 meters below the surface, was a colossal stone structure that defied easy explanation. Dubbed the Yonaguni Monument, the primary formation is a massive rectangular block of rock measuring approximately 150 meters long by 40 meters wide, and rising to a height of 27 meters. Its top sits just 5 meters below the ocean surface.
The structure is composed of medium-to-fine sandstone and mudstone from the Lower Miocene Yaeyama Group, deposited around 20 million years ago. Its features include: expansive flat terraces, seemingly carved steps, and near-perfect right angles that suggest artificial construction. Specific named features include a 5-meter-wide ledge called the "Loop Road," a triangular depression known as the "Triangle Pool," and two towering stone pillars that rise to within a few meters of the surface. Most of the monument is connected to the underlying rock mass, meaning it was likely carved from a single piece, not assembled from separate blocks.
The Geologic and Archeologic Debate
The origin of the monument is a subject of scientific debate. Masaaki Kimura, a marine geologist at the University of the Ryukyus, has studied the site for years and argues that it is a man-made monolith. Kimura proposes the structure is at least 5,000 years old, dating to a time when lower sea levels would have exposed the area as dry land. He has identified features he interprets as a castle, temples, a stadium, and a triumphal arch, all connected by what appear to be roads and waterways. The main argument for an artificial origin is the high concentration of precise, geometric features in a relatively small area.
Geologist Robert Schoch of Boston University argues that the monument is a natural formation. He explains that the area's sandstone has well-defined parallel bedding planes and is crisscrossed by vertical joints from tectonic activity. This combination causes the rock to fracture in clean, straight lines, creating the step-like terraces and right angles. Schoch points to similar, albeit smaller, natural formations on the coastline of Yonaguni island itself as evidence. The strong ocean currents in the area also contribute to the sharp erosion. The Japanese government's Agency for Cultural Affairs has not designated the monument as an important cultural artifact, and has not conducted its own research or preservation work at the site.
💡Fun Facts
A nearby feature is nicknamed "the Turtle" due to its distinct, carved-looking shape when viewed from above.
The waters around Yonaguni are famous for large populations of hammerhead sharks, which was the reason the monument's discoverer was exploring the area in the first place.
The island of Yonaguni is geographically closer to Taiwan (about 111 kilometers away) than it is to mainland Okinawa.
Some proponents of the artificial origin theory believe the site could be a remnant of the mythical lost continent of Mu, a Pacific version of Atlantis.
Accessible year-round via licensed dive boat operators; tours typically run during daylight hours, weather permitting.
Admission
The site is free to view from a boat, but accessing it underwater requires booking a paid dive tour.
Accessibility
This is an open-water dive site. Due to strong currents and depth, it is recommended only for experienced, advanced-certified divers. Some operators offer semi-submersible boat tours for non-divers.
