Underwater Wall off Ile de Sein, Brittany, 7000BP

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French marine archaeologists have discovered a large underwater stone wall off the coast of the Île de Sein, at the western tip of Brittany, dating to around 5,000 BC. Measuring approximately 120 metres in length, the wall is the largest underwater construction ever identified in France. When the wall was built it would have stood along the shoreline, between the high- and low-tide marks. Today, it lies beneath nine metres of water. Archaeologists believe it may have functioned either as a fish trap or as a dyke designed to protect against encroaching sea levels.

" The wall is on average 20 metres wide and two metres high. At regular intervals divers found large granite standing stones – or monoliths – protruding above the wall in two parallel lines. It is believed these were originally placed on the bedrock and then the wall built around them out of slabs and smaller stones. If the fish-trap hypothesis is the right one, then the lines of protruding monoliths would have also supported a "net" made of sticks and branches to catch fish as the tide retreated. With an overall mass of 3,300 tonnes, the wall must have been the work of a substantial settled community [...] "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," said archaeologist Yvan Pailler. "

The structure was first identified after local geologist Yves Fouquet examined high-resolution seabed charts produced using modern radar technology. Just off Sein, he noticed a 120-metre line blocking an underwater valley that could not have been natural. Initial archaeological dives took place in the summer of 2022, but detailed mapping had to wait until the following winter, when reduced seaweed growth improved visibility.