In 1985, off the coast of Yonaguni Island, located near Okinawa, Japan, underwater tour leader Kihachiro Aratake discovered several large stone pyramidal structures. However, it was not until 1997 that investigations, led by Masaaki Kimura, attracted media attention. Initial tests have concluded that the age of the pyramids dates back to around 10,000 years ago. With the evidence found in these pyramids, along with the discovery of walls and ancient roads in the Straits of Taiwan, many scholars are proposing advanced societies that existed during around 8,000 B.C.
There are a variety of different explanations for these structures. One, which is not widely believed, is that these are some of the remains of Atlantis, or Lemuria. Other more probable, or at least more popular beliefs include that the pyramids were simply man made structures that belonged by a previous civilization that happened to "sink" into the ocean as global water levels rose after the last ice age. The degree of human influence on the structures, however, cannot be widely agreed upon. Many scientists believe that the pyramids were completely man made, showing signs of wide terraces, ramps, and steps. However, most American scientists studying these ruins believe that the pyramids are only an example of early terra forming, where man had taken the land and only slightly modified the terrain to better suit his needs. Despite all of that, it is also a possibility that the pyramids are nothing more than geometrical rock formations, that randomly came to be, much like the steps of Finn McCool, located in Ireland (thanks Roninspoon).
While there is still much debate on how, and why these pyramids came to be, or more importantly, who built them, Professor Masaaki Kimura is dedicated to proving his hypothesis of a sunken civilization.
http://www.summit-okinawa.gr.jp/tokusyu/ruins1.htm
http://www.morien-institute.org/yonaguni.html