French Divers Find Underwater Wall From 5,000 BC

A long, stony line on the seabed off western France is turning out to be a lot more than a geological quirk. Marine archaeologists say a 394-foot structure found off the Ile de Sein in Brittany is a man-made wall dating to roughly 5,000 BC, making it the largest known underwater construction in France, the BBC reports. Now 30 feet below the surface, the wall would originally have sat in the tidal zone, when the island's coastline stretched much farther out. Researchers think it was either a large fish trap or an early attempt to hold back the sea.

The structure averages 65 feet wide and about 7 feet high, with paired granite monoliths sticking up at intervals in two rows. Those standing stones appear to have been set on bedrock first, with the wall later built around them from slabs and smaller rocks. If it was a fishing device, archaeologists say the uprights likely held a mesh of branches and stakes to catch fish as the tide ebbed. With an estimated mass of 3,300 tons, it points to a sizable, organized community—either sedentary hunter-gatherers or early Neolithic farmers.

The monoliths resemble, but predate, Brittany's famous menhirs—standing stones found throughout the countryside—suggesting stone-working techniques may have been passed between early communities. The wall was first spotted when geologist Yves Fouquet noticed a straight, unnatural-looking line "blocking off an undersea valley" on new high-resolution depth charts. A team surveyed the site in 2022 and returned after winter die-back of seaweed to map it in full.

In a study published in the the International Journal of Nautical Archaeology, the authors argue that such drowned landscapes may underlie Breton legends of lost cities like Ys, said to have been in a bay a few miles to the east of the Ile de Sein. "It is likely that the abandonment of a territory developed by a highly structured society has become deeply rooted in people's memories," researchers wrote. "The submersion caused by the rapid rise in sea level, followed by the abandonment of fishing structures, protective works, and habitation sites, must have left a lasting impression."