Gates goes to the ruins of Yonaguni Island, the westernmost Japanese island on the East China Sea, to determine whether the undersea structures are geologic or man-made. The Yonaguni Monument discoverer, Kihachiro Aratake, dives with Josh, showing him the undersea area with cut steps leading up to a level platform. Josh takes a rock sample for geologic analysis. In Osaka, archaeologist Michio Maezano shows Josh an ancient rock tomb. It proves that ancient people could have carved the Yonaguni structures, then an earthquake could have sunk them.