Welcome to Yonaguni-jima
In 1985 a local diver discovered what appeared to be human-made 'ruins' off the south coast of the island, which came to be known as the Kaitei Iseki (海底遺跡), or Yonaguni Monument. Some claim the ruins, which look like the steps of a sunken pyramid, are the remains of a Pacific Atlantis, though science suggests they're the result of natural geological processes. Local dive-school operators can get you down there.
Adding to the underwater allure are the large schools of hammerhead sharks that frequent the waters off the west coast, making Yonaguni perhaps the most famous single diving destination in Japan.