Discovered in 1985 by a local diver, these underwater formations off Yonaguni's south coast look for all the world like the steps of a sunken pyramid — massive, flat-topped blocks arranged in geometric patterns that seem impossible to explain by geology alone. Some believe them to be the remains of a Pacific Atlantis; others insist they're natural. Adding to the mystique, large schools of hammerhead sharks patrol the waters off the west coast in winter. Whether you believe in the ruins or just the sharks, diving here is a pilgrimage for underwater adventurers — perhaps the single most famous dive destination in all of Japan.