Marine archaeologists in France have discovered a massive 7,000-year-old wall underwater off the western coast of Brittany. The 120-metre-long granite wall near the Ile de Sein is the largest underwater construction ever found in France. Alongside it, they found a dozen smaller manmade structures from the same time. The discovery, published in the International Journal of Nautical Archaeology, offers new insights into early coastal settlements and how they faced rising sea levels. "This is a very interesting discovery that opens up new prospects for underwater archaeology, helping us better understand how coastal societies were organised," said Yvan Pailler, archaeology professor at the University of Western Brittany and co-author of the report. The wall was first spotted in 2017 by retired geologist Yves Fouquet on undersea charts made with laser technology. Between 2022 and 2024, divers confirmed the granite structures' presence. "Archaeologists did not expect to find such well-preserved structures in such a harsh setting," Fouquet said. Dating between 5,800 and 5,300 BC, the wall lies about nine metres underwater. When built, sea levels were lower so the site was on the shoreline between high and low tide. Experts think it may have been used as a fish trap or a dyke to guard against rising seas. The wall is about 20 metres wide and two metres high. Large granite blocks stick out in two parallel rows, possibly to hold nets made from sticks if it was a fish trap. Weighing roughly 3,300 tonnes, the structure needed a well-organized group to build it. Pailler added, "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 BC." The BBC also notes the blocks are older than the famous Neolithic menhirs of Brittany, showing stone-working knowledge spread from Mesolithic hunter-gatherers to Neolithic farmers. Researchers think these underwater sites may have inspired local legends of sunken cities, like the mythical city of Ys in the Bay of Douarnenez nearby.