A Massive Stone Wall Built 7,000 Years Ago Was Found Intact Beneath the Sea Off the Coast of France

An underwater find in Brittany reveals how ancient people faced a rising sea.

Seven thousand years ago, people living on the Atlantic edge of Europe built a massive wall of stone where land met water. Today, that wall lies nearly nine meters underwater, still intact, stretched across a drowned valley off the coast of Brittany.

Marine archaeologists say it is the largest underwater stone construction ever identified in France. Detailed in a new study in the International Journal of Nautical Archaeology, the structure dates to between about 5,800 and 5,300 BCE, when melting ice sheets were rapidly raising global sea levels.

Discovered near the Île de Sein at Brittany’s western edge, the wall reveals how prehistoric coastal people shaped their surroundings and coped with rising seas.

A Line on the Seabed

In 2017, retired geologist Yves Fouquet was studying high-resolution seabed charts created using lidar when he noticed something odd: a straight line cutting across an underwater valley.

“Just off Sein, I saw this 120-meter line blocking off an undersea valley. It didn’t make sense from a geologic point of view,” Fouquet told the BBC.

Between 2022 and 2024, teams of divers returned to investigate. They confirmed that the feature was a human-built wall, 120 meters long, about 20 meters wide, and up to two meters high. It lies in an area swept by strong currents and heavy swells, conditions that make underwater work difficult and preservation unlikely.

Yet the structure has endured. Granite blocks form the body of the wall, while large standing stones (monoliths) rise from its crest in two parallel rows. Some still stand more than a meter above the surrounding stone.

When the wall was built, it sat directly on the shoreline, between high and low tide. Since then, the sea level around Brittany has risen by roughly 25 meters as the last ice age ended. The Île de Sein itself has shrunk dramatically, leaving the wall submerged beneath cold, fast-moving water.

The researchers identified at least 11 stone structures in the area, including smaller walls that partially block nearby channels.

Who Built It, and Why?

The size and complexity of the wall raise fundamental questions about the ancient builders.

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

Large stone constructions are often linked to farming societies, which could rely on stored food and settled labor. But coastal hunter-gatherers, the study argues, may have enjoyed rich and predictable marine resources that allowed them to settle and collaborate on large projects.

The wall’s function remains debated. One possibility is that it served as a fish trap. Stone fish weirs are known across prehistoric Europe, designed to funnel fish into enclosures as tides recede. The upright stones on the wall could once have supported a lattice of wooden branches.

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Several nearby structures fit this pattern. But the main wall is unusually large. Its height and width exceed what would normally be required for trapping fish, given the local tidal range.

The researchers outline another possibility: that the wall also served as a protective barrier, essentially a dyke, shielding low-lying land from waves and encroaching seas. Its architecture supports that idea. The wall is asymmetrical, with extra mass on the side exposed to the strongest swells. The largest monoliths are deeply anchored and remain upright despite thousands of years of storms.

Before the Floods

The wall was built during a period of rapid environmental change. At the end of the last ice age, global sea levels rose quickly as ice sheets melted. Coastlines shifted inland, drowning large areas of habitable land.

The team used detailed reconstructions of past sea levels to estimate when the structures were built. Both the fish-trap and protective-wall interpretations place construction during a time when the sea was rising fast.

Similar submerged sites are now being found elsewhere in northern Europe, revealing a broader pattern of drowned coastal worlds. In Denmark, archaeologists recently excavated a Stone Age settlement preserved beneath the sea. “It’s like a time capsule,” underwater archaeologist Peter Moe Astrup told the Associated Press about the separate discovery. “When sea level rose, everything was preserved in an oxygen-free environment… time just stops.”

The Brittany wall hints at the human consequences of those changes. The study’s authors suggest that the loss of engineered coastal landscapes may have left a deep cultural imprint, possibly inspiring local legends of sunken cities, including the mythical city of Ys.

“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 write.

The ancient sea-level rise that submerged the wall was driven by natural climate shifts. Today, rising seas are largely the result of human activity.