The Yonaguni Monument, also called the Yonaguni Submarine Ruins, is an underwater rock structure located off the coast of Yonaguni Island, which is the southernmost island in the Ryukyu Islands of Japan. It is about 100 kilometers (54 nautical miles; 62 miles) east of Taiwan.

Marine geologist Masaaki Kimura believes the formations are man-made stepped monoliths. However, some experts do not agree with this view. The Japanese Agency for Cultural Affairs and the government of Okinawa Prefecture do not recognize the site as important cultural artifacts. Neither agency has conducted research or preservation efforts at the location.

Discovery

The sea near Yonaguni is a well-known place for diving in the winter months because many hammerhead sharks live there. In 1986, Kihachiro Aratake, who worked for the Yonaguni-Cho Tourism Association, saw unusual shapes on the ocean floor while looking for sharks. These shapes looked like buildings or structures made by people. Soon after, a team of scientists led by Masaaki Kimura from the University of the Ryukyus visited the same area to study the formations.

Location and geology

These formations are found at the base of the Arakawabana cliff, which is the southern end of Yonaguni Island. The main side of the cliff faces south-southeast.

The formations are made up of medium to very fine sandstone and mudstone from the Early Miocene Yaeyama Group. These rocks were formed about 20 million years ago. Most of the rocks in these formations are attached to the rock below, rather than being made up of separate, freestanding rocks.

Natural formation

Geologist Robert M. Schoch believes the Yonaguni formation is most likely natural. He studied the sandstone layers of the formation and found that they have many clear, parallel layers that can easily be separated. The rocks also have many sets of parallel, vertical cracks, which are natural fractures. These cracks may have helped create the rectangular shapes seen in the area. Yonaguni is in a region where earthquakes often occur, and these events can cause rocks to break in regular patterns. Schoch also noted similar formations on the northeast coast of Yonaguni. John Anthony West, who visited the site with Schoch, agreed that the formation is natural and said Kimura did not examine the natural processes carefully enough. Schoch believes the "drawings" Kimura identified are natural scratches on the rocks. He also says the "walls" are natural horizontal platforms that tilted into a vertical position when the rock below them eroded. He suggests the "roads" are simply channels in the rock.

German geologist Wolf Wichmann studied the Yonaguni formations in 1999 and 2001 and concluded they could have formed through natural processes.

At Sanninudai, there are similar step-like sandstone formations on land. Robert Schoch and Patrick D. Nunn, a professor of oceanic geoscience, stated these formations are entirely natural.

In 2019, Takayuki Ogata and other researchers used a digital elevation model and studied rock layers, rocks, and small surface features at three locations on Yonaguni Island. Their findings showed that while the Yonaguni Monument may look like an artificial structure, it is a natural feature shaped by weathering and erosion acting on sandstone layers and cracks. They noted similar features at Sanninudai and on the south coast of Yonaguni Island.

Kimura first estimated the formation is at least 10,000 years old, suggesting it was above water during that time and might be a remnant of the mythical lost continent of Mu. In 2007, he revised his estimate to 2,000 to 3,000 years ago, stating that sea levels were similar to today’s. He claimed tectonic activity caused the site to sink underwater after construction. Archaeologist Richard J. Pearson believes this is unlikely. Kimura identified structures like a pyramid, castles, roads, monuments, and a stadium, and suggested they are remnants of the Yamatai culture.

Supporters of an artificial origin, such as Graham Hancock, argue that while some Yonaguni features resemble natural sandstone formations worldwide, the high number of unusual structures in such a small area is unlikely. They also note the lack of loose rock blocks on flat areas, which would be expected if erosion and fracturing were the only causes. Schoch, who believes the formation is geological, pointed out that strong ocean currents move the rocks.

Visitors

The formation has become a well-known attraction for divers even though there are strong ocean currents. A famous visitor was freediver Jacques Mayol, who wrote a book about his dives at Yonaguni.