A mysterious underwater structure off the coast of Japan has some experts wondering if we've seriously underestimated ancient civilizations.

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Known as the Yonaguni Monument, 82 feet (24.99 meters) below the surface near Japan’s Ryukyu Islands lies an ancient mystery that could potentially change what we know about ancient history.

An underwater pyramid

Shaped like a massive stepped pyramid, the ancient structure rises nearly 90 feet (27.43 meters) from the sea floor — and appears to be made entirely of stone.

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Tests suggest the pyramid is made up of over 10,000 years old stone. If it is really man-made, that would mean an unknown civilization built it long before sea levels rose after the last Ice Age — that too thousands of years before the Egyptian pyramids (5,000 years before) or Stonehenge (6,000 years before).

What's fuelling the debate? 

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As per the Daly Mail, authors like Graham Hancock have gone as far as to compare it to the mythical Atlantis. 

"It's stunning," Hancock said in a recent debate on The Joe Rogan Experience, pointing to what he claims are "human-made arches, megaliths, steps, terraces, and what seem to be a carved rock 'face'."

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But many experts remain unconvinced. Archaeologist Flint Dibble, who also appeared on the podcast, argued the whole structure looks natural to him. "I've seen a lot of crazy natural stuff and I see nothing here that to me reminds me of human architecture," he said.

Geologist Dr Robert Schoch agrees. In 1999, he said that he believes the formation was shaped by earthquakes and erosion — not human hands. Yonaguni, he says, shares features with other natural rock formations nearby and sits in a tectonically active zone.

Still, others — including Japanese geologist Dr Masaaki Kimura, who dated the rocks — say the case isn't that straight forward. 

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If proven to be artificial, the Yonaguni Monument could join other ancient enigmas like Göbekli Tepe in Turkey (around 11,000 years old) and Gunung Padang in Indonesia, which some researchers claim could be as old as 16,000 years.

All of them raise big questions: Were humans building giant stone structures far earlier than we thought? And if so, what else have we missed beneath the waves — or buried in plain sight?

For now, Yonaguni remains a mystery — part geology, part archaeology, and part legend.

(With inputs from agencies)