Submerged stone structures lying just below the waters off Yonaguni Jima are actually the ruins of a Japanese Atlantis—an ancient city sunk by an earthquake about 2,000 years ago.
That's the belief of Masaaki Kimura, a marine geologist at the University of the Ryukyus in Japan who has been diving at the site to measure and map its formations for more than 15 years.
Each time he returns to the dive boat, Kimura said, he is more convinced than ever that below him rest the remains of a 5,000-year-old city.
"The largest structure looks like a complicated, monolithic, stepped pyramid that rises from a depth of 25 meters [82 feet]," said Kimura, who presented his latest theories about the site at a scientific conference in June.
But like other stories of sunken cities, Kimura's claims have attracted controversy.
"I'm not convinced that any of the major features or structures are manmade steps or terraces, but that they're all natural," said Robert Schoch, a professor of science and mathematics at Boston University who has dived at the site.
"It's basic geology and classic stratigraphy for sandstones, which tend to break along planes and give you these very straight edges, particularly in an area with lots of faults and tectonic activity."
And neither the Japanese government's Agency for Cultural Affairs nor the government of Okinawa Prefecture recognize the remains off Yonaguni as an important cultural property, said agency spokesperson Emiko Ishida.
Neither of the government groups has carried out research or preservation work on the sites, she added, instead leaving any such efforts to professors and other interested individuals.
Yonaguni Jima is an island that lies near the southern tip of Japan's Ryukyu archipelago, about 75 miles (120 kilometers) off the eastern coast of Taiwan (see map).
A local diver first noticed the Yonaguni formations in 1986, after which a promontory on the island was unofficially renamed Iseki Hanto, or Ruins Point.
The district of Yonaguni officially owns the formations, and tourists and researchers can freely dive at the site.
Some experts believe that the structures could be all that's left of Mu, a fabled Pacific civilization rumored to have vanished beneath the waves.
On hearing about the find, Kimura said, his initial impression was that the formations could be natural. But he changed his mind after his first dive.
"I think it's very difficult to explain away their origin as being purely natural, because of the vast amount of evidence of man's influence on the structures," he said.
For example, Kimura said, he has identified quarry marks in the stone, rudimentary characters etched onto carved faces, and rocks sculpted into the likenesses of animals.
"The characters and animal monuments in the water, which I have been able to partially recover in my laboratory, suggest the culture comes from the Asian continent," he said.
"One example I have described as an underwater sphinx resembles a Chinese or ancient Okinawan king."
Whoever created the city, most of it apparently sank in one of the huge seismic events that this part of the Pacific Rim is famous for, Kimura said.
The world's largest recorded tsunami struck Yonaguni Jima in April 1771 with an estimated height of more than 131 feet (40 meters), he noted, so such a fate might also have befallen the ancient civilization.
Kimura said he has identified ten structures off Yonaguni and a further five related structures off the main island of Okinawa. In total the ruins cover an area spanning 984 feet by 492 feet (300 meters by 150 meters).
The structures include the ruins of a castle, a triumphal arch, five temples, and at least one large stadium, all of which are connected by roads and water channels and are partly shielded by what could be huge retaining walls.
Kimura believes the ruins date back to at least 5,000 years, based on the dates of stalactites found inside underwater caves that he says sank with the city.
And structures similar to the ruins sitting on the nearby coast have yielded charcoal dated to 1,600 years ago—a possible indication of ancient human inhabitants, Kimura added.
But more direct evidence of human involvement with the site has been harder to come by.
"Pottery and wood do not last on the bottom of the ocean, but we are interested in further research on a relief at the site that is apparently painted and resembles a cow," Kimura said.
"We want to determine the makeup of the paint. I would also like to carry out subsurface research."
Toru Ouchi, an associate professor of seismology at Kobe University, supports Kimura's hypothesis.
Ouchi said that he has never seen tectonic activity having such an effect on a landscape either above or below the water.
"I've dived there as well and touched the pyramid," he said. "What Professor Kimura says is not exaggerated at all. It's easy to tell that those relics were not caused by earthquakes."
Boston University's Schoch, meanwhile, is just as certain that the Yonaguni formations are natural.
He suggests that holes in the rock, which Kimura believes were used to support posts, were merely created by underwater eddies scouring at depressions.
Lines of smaller holes were formed by marine creatures exploiting a seam in the rock, he said.
"The first time I dived there, I knew it was not artificial," Schoch said. "It's not as regular as many people claim, and the right angles and symmetry don't add up in many places."
He emphasizes that he is not accusing anyone of deliberately falsifying evidence.
But many of the photos tend to give a perfect view of the site, making the lines look as regular as possible, he said.
Schoch also says he has seen what Kimura believes to be renderings of animals and human faces at the site.
"Professor Kimura says he has seen some kind of writing or images, but they are just scratches on a rock that are natural," he said.
"He interprets them as being manmade, but I don't know where he's coming from."
But Kimura is undeterred by critics, adding that the new governor of Okinawa Prefecture and officials from the United Nations Educational, Scientific, and Cultural Organization have recently expressed interest in verifying the site.
"The best way to get a definitive answer about their origins is to keep going back and collecting more evidence," he continued.
"If I'd not had a chance to see these structures for myself, I might be skeptical as well."
A structure thought to be the world's oldest building, nearly twice the age of the great pyramids of Egypt, has been discovered. The rectangular stone ziggurat under the sea off the coast of Japan could be the first evidence of a previously unknown Stone Age civilization, say archeologists.
The monument is 600ft wide and 90ft high and has been dated to at least 8000BC. The oldest pyramid in Egypt, the Step Pyramid at Saqqara, was constructed more than 5,000 years later.
The structure off Yonaguni, a small island southwest of Okinawa, was first discovered 75ft underwater by scuba divers 10 years ago and locals believed it was a natural phenomenon.
Professor Masaki Kimura, a geologist at Ryukyu University in Okinawa, was the first scientist to investigate the site and has concluded that the mysterious five-layer structure was man-made. "The object has not been manufactured by nature. If that had been the case, one would expect debris from erosion to have collected around the site, but there are no rock fragments there," he said.
The discovery of what appears to be a road surrounding the building was further evidence that the structure was made by humans, he added.
Robert Schoch, professor of geology at Boston University, dived to the site, "It basically looks like a series of huge steps, each about a meter high. Essentially, it's a cliff face like the side of a stepped pyramid. It's a very interesting structure," he said. "It's possible that natural water erosion combined with the process of cracked rocks splitting created such a structure, but I haven't come across such processes creating a structure as sharp as this."
Further evidence that the structure man-made came with the discovery of smaller underwater stone mounds nearby. Like the main building, these mini-ziggurats are made of stepped slabs and are roughly 10m wide and 2m high.
Kimura said it was too early to know who built the monument or its purpose. "The structure could be an ancient religious shrine, possibly celebrating an ancient deity resembling the god Nirai-Kanai, whom locals say gave happiness to the people of Okinawa from beyond the sea. This could be evidence of a new culture as there are no records of a people intelligent enough to have built such a monument 10,000 years ago," he said.
"This could only have been done by a people with a high degree of technology, probably coming from the Asian continent, where the oldest civilizations originate. There would have to have been some sort of machinery involved to have created such a huge structure."
Teruaki Ishii, professor of geology at Tokyo University, said the structure dated back to at least 8000BC when the land on which it was constructed was submerged at the end of the last ice age. "I hope this site is artificial as it would be very exciting. But at this time I feel it is too early to say. I think the structure could be natural, but part of it may have been made," he said.
The first signs of civilization in Japan are traced to the Neolithic period around 9000BC. The people at this time lived as hunters and food- gatherers. There is nothing in the archeological record to suggest the presence of a culture advanced enough to have built a structure like the ziggurat.
Jim Mower, an archeologist at University College London, said: "If it is confirmed that the site is as old as 10,000 years and is man-made, then this is going to change an awful lot of the previous thinking on southeast Asian history. It would put the people who made the monument on a par with the ancient civilization of Mesopotamia and the Indus Valley."