Underwater stepped structures with suspiciously right angles lie off Japan's coast. Natural sandstone erosion or a sunken pyramid? Mainstream geologists say nature, a vocal minority insists these are ruins of a lost civilization.
Melkov, CC0, via Wikimedia Commons
A geometric anomaly
Discovered by chance in 1986 by a diver searching for hammerhead shark viewing spots, the Yonaguni Monument is a massive submerged rock formation that is still debated. The main structure is a rectangular monolith measuring approximately 150 meters long by 40 meters wide, rising to a height of about 27 meters. Its top surface, which lies just 5 meters below sea level, consists of a series of broad, flat terraces and steps bounded by near-vertical walls.
The entire formation is composed of sandstone and mudstone from the Miocene Yaeyama Group, deposited around 20 million years ago. Its peculiar features include its seemingly non-natural features: numerous straight edges, sharp corners that appear to be 90-degree angles, and large, flat platforms. Specific features identified by observers include a 5-meter wide ledge called the "Loop Road" and a triangular depression known as the "Triangle Pool". The structure is not assembled from separate blocks; it is one continuous piece connected to the underlying rock mass.
The geological debate
The central question is whether these geometric shapes are natural or artificial. The mainstream geological consensus points to natural processes. Robert Schoch, a geologist from Boston University, argues that the monument's features are the result of "basic geology and classic stratigraphy". Sandstone is a sedimentary rock that forms in layers and tends to break along straight planes, especially in a region with significant fault lines and tectonic activity like Yonaguni. He notes that the numerous parallel bedding planes and vertical joints in the rock naturally cause rectangular fracturing. Similar formations can be found on the coast of Yonaguni island itself. The strong ocean currents in the area could also contribute to the uniform erosion that sharpens these natural breaks.
Marine geologist Masaaki Kimura of the University of the Ryukyus has been a principal proponent of the artificial origin theory. He argues the concentration of so many geometric shapes in one area is unlikely to be natural. Kimura has identified what he interprets as tool marks, carvings resembling animals, and structures he calls a pyramid, castles, and a stadium. If the structure was modified by humans, it would have to have been done during the last ice age, around 10,000 BCE, when this area was above sea level. This timeline would place its construction before other known monumental architecture.
💡Fun Facts
The monument was discovered by Kihachiro Aratake, who was looking for new spots to show tourists hammerhead sharks.
Some proponents of the artificial theory connect the structure to the legend of the lost continent of Mu, a mythical Pacific civilization similar to Atlantis.
The powerful and persistent Kuroshio Current flows through the waters around Yonaguni, a factor geologists say could explain the sharp, eroded features.
Native Yonaguni horses, a critically endangered breed, roam freely on the island above the underwater site.
The site is in open water and accessible only through guided boat and dive tours, which typically operate during daylight hours, weather permitting.
Admission
Prices for guided dives start around 12,000-15,000 JPY per person, including equipment rental and transportation to the site.
Accessibility
Accessing the monument requires diving. Due to strong currents and depth (5 to 40 meters), it is recommended only for advanced divers with experience in drift and deep diving. Some operators require a minimum of 100 logged dives.
