Marine archaeologists in France have discovered the underwater ruins of massive stone structures dating back to at least 5300 B.C.E. off the coast of Brittany.

These ruins are among the oldest large stone structures ever found in France. The research team, which published its findings in the International Journal of Nautical Archaeology, furthermore thinks that the ruins may have come from a Stone Age civilization that inspired a local legend about an ancient sunken city known as Ys.

The Discovery Of Massive Submerged Ruins Off The Coast Of Brittany
The 7,000-year-old submerged granite ruins vary in size, the largest of which is a wall that’s 394 feet long and was likely used as a fish trap or a dyke to shield against rising sea levels, archaeologists said.

On average, the wall is around 65 feet wide and protrudes about six feet upward from the seafloor.

When it was constructed, however, it would have been on the shoreline of Sein Island – where the ruins were found, along Brittany’s western tip – between the low and high tide marks. As the island has shrunk over the centuries, the ruins now sit below 30 feet of water.

The wall is also marked by stone monoliths at regular intervals, which could support the fish trap hypothesis, if the standing stones were meant to hold nets in place.

Whatever their purpose, the existence of these structures raises intriguing questions about the people who built them.

“It was built by a very structured society of hunter-gatherers, of a kind that became sedentary when resources permitted. That or it was made by one of the Neolithic populations that arrived here around 5,000 B.C.E.,” archaeologist Yvan Pailler told the BBC.

Breton legends about sunken cities were fairly common in ancient times, but the ruins suggest that these legends may have had some basis in reality.

“It is likely that the abandonment of a territory developed by a highly structured society has become deeply rooted in people’s memories,” the researchers wrote in their paper.

As sea levels rose rapidly and people were forced to abandon their settlements, they may very well have spread the story. An ancient game of telephone could have then warped the truth of that story into something more fantastical over time.

Sunken French City Near Brittany
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The ruins may have partly inspired legends about the Lost City of Ys.

“Legends about sunken cities, compared with recent data on rising sea levels, shows that the stories of ancient submergences, passed down by oral tradition, could date back as far as 5,000 to 15,000 years,” the researchers noted.

“This suggests that oral traditions that may have preserved significant events in memory that could well be worthy of scientific examination. These settlements described in legend reveal the profound symbolic significance of maritime prehistory, which should not be overlooked.”

Some of the most popular tales of sunken cities in Brittany involve the Lost City of Ys, which is thought to be located in the Bay of Douarnenez, roughly six miles east of Sein Island. The new findings could provide reason to re-evaluate those legends, however, with new insight into their origins.

And it’s all thanks to a chance discovery made by one geologist.