In 1987, a local diver exploring off the coast of Japan's southern Ryukyu Islands stumbled across a startling discovery. Twenty-five metres below the surface, he spotted a series of almost perfectly carved steps with straight edges. Known today as the Yonaguni Monument, this massive 50m-long-by-20m-wide behemoth is one of the world's most unusual underwater sites. Nicknamed "Japan's Atlantis", the rectangular, stacked pyramid-like monument is believed to be more than 10,000 years old. Some think it's all that remains of a long-lost Pacific civilisation, possibly built by Japan's prehistoric Jomon people who inhabited these islands as early as 12000 BC. Others say that the site resembles natural formations elsewhere around the world with distinctly defined edges and flat surfaces, such as Northern Ireland's Giant's Causeway, whose thousands of interlocking basalt columns were formed by a volcanic eruption millions of years ago. Marine geologist Masaaki Kimura claims that the formations are man-made stepped monoliths.[1] These claims have been described as pseudoarchaeological.[2] Neither the Japanese Agency for Cultural Affairs nor the government of Okinawa Prefecture recognise the features as important cultural artifacts and neither government agency has carried out research or preservation work on the site.