Ancient megalithic construction beneath the sea off Ryukyu islands in Japan, submerged by post glacial sea-level change

SEABAT sonic sounding, underwater robot and SCUBA diving surveys reveal the existence of artificial topographies constructed about 10,000 years ago beneath the sea off the Ryukyu islands in Japan. Features were found that look like an ancient city including stepped pyramids, roads and water canals off of the southern coast of Yonaguni Island. Its width is about 1 km×1 km and water depths vary from 0 to 35 m. Monuments are connected by roads constructed by cutting the basement rock or laying flagstones. The most elaborate one in the point is labeled as No. 1 Monument or Yonaguni Underwater Pyramid (YUP). It looks like a stepped pyramid which closely resembles a giant Gusuku from ancient Okinawa. The Gusuku is thought to be a mixed structure, part castle and part temple.