The Yonaguni Monument

The Yonaguni Monument, off the coast of Japan's most westerly island, consists of a geometric terraced structure 27 metres high with vertical walls, as well as a number of other structures resembling trenches, paths, platforms and megaliths.

The site is well known among scuba divers but there is a lack of consensus over whether the site is a natural phenomenon, or whether it could in fact be the submerged remained of an ancient city.

If the site was man-made (or at least man-enhanced), it could, controversially, suggest that people were living on Yonaguni 10,000 years ago.