In 1985, Kihachiro Aratake, a dive tour operator in Yonaguni, Japan was scouting for a new site to view hammerhead sharks.   He had dropped into the water on the southeast side of the small island, off a place called Arakawa Point, expecting to see the normal crevices, coral heads, and possibly an entrance to an underwater cave.   What lay before him, however, was something that no one had viewed in thousands of years.....