The History Channel series History's Mysteries recently broadcast a program on "Japan's Mysterious Pyramids." The remarkable stone structures referred to are near Yonaguni in Okinawa. Wonder Okinawa offers a site with quite a few images and details about these structures.
http://www.wonder-okinawa.jp/024/english/
The structure was discovered by a diver in 1985. It is about 100 m long, 30 m wide, and at its highest point is about 23 m tall. Some argue that the structure was erected during the last ice age, or perhaps 12,000 years ago.
Not everyone, however, is convinced that the structure is man-made, despite the many square edges that are evident. In 1998, Boston University geologist Robert Schoch told the Australian paper the Sunday Mail "I think the evidence is tantalising but not conclusive or definitive."