Divers examine the base of a stone structure off the coast of Yonaguni Jima in Japan's Ryukyu archipelago in an undated photo.

Some experts argue that the stone steps are human-made and that they are part of a vast sunken city akin to Atlantis.

Tests on stalactites found near the rock formations indicate that the site is about 5,000 years old, said the University of the Ryukyus' Masaaki Kiruma. Other features above and below water indicate that the structures sank beneath the waves about 2,000 years ago, he maintains.

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