September 19, 2007—Not far from Taiwan, Pacific waters engulf stone structures just off the coast of the tiny Japanese island of Yonaguni Jima, part of the Ryukyu archipelago in this undated photo.

The structures maybe the ruins of a 5,000-year-old city that featured a castle, several temples, and a stadium, according to a Japanese researcher who has been diving at the site for past 15 years. (Read the full story.)

"The largest structure looks like a complicated, monolithic, stepped pyramid that rises from a depth of 25 meters [82 feet]," said the scientist, Masaaki Kimura of the University of the Ryukyus. Kimura recently suggested that the site might have been sunk by a massive tsunami similar to the one that hit the island in 1771.

But other experts who have dived at the site, which was discovered in 1986, are equally convinced that the formations are natural.

"It's basic geology and classic stratigraphy for sandstones, which tend to break along planes and give you these very straight edges," said Robert Schoch, a professor of science and mathematics at Boston University and a leading critic of the theory.

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