NAHA, Okinawa Pref. -- In the waters off remote Yonaguni Island, from which Taiwan can be seen on a clear day, lies one of Japan's most puzzling mysteries.

A giant stone structure with perfect geometric angles that appear to be tunnels and staircases has convinced some, including noted writer Graham Hancock, that an unknown ancient civilization occupied the area nearly 10,000 years ago.

But others, including a noted geologist, are skeptical, saying the geometrical shapes of the stones may be due to a unique, but natural, form of erosion.