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