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.
There's another article and video here.
I don't know. From some angles it looks like it could be artificial and from others, it looks like it could be natural.