Divers have discovered a wall submerged for some 7,000 years under the sea off the coast of western France, scientists announced Thursday.
Nearly 400 feet long, it was discovered off the coast of the island of Sein in Brittany along with a dozen smaller man-made structures dating from the same period.
“This is a very interesting discovery which opens up new perspectives for underwater archaeology, allowing us to better understand the organization of coastal societies,” Yvan Pailler, professor of archeology at the University of Western Brittany, told AFP.
He co-authored a study on this discovery, published in the International Journal of Nautical Archaeology.
The structures were first spotted by retired geologist Yves Fouquet in 2017 on seabed maps made with a laser system.
The wall is the largest underwater construction ever discovered in France, BBC News reported.
SAMM, 2023/Yves Fouquet et al. International Journal of Nautical Archeology (2025)
Divers explored the site between 2022 and 2024 and confirmed the presence of the granite structures.
“Archaeologists did not expect to find such well-preserved structures in such a hostile environment,” Fouquet said.
Dating from 5,800 to 5,300 BC, they sit nine meters underwater and were built at a time when sea levels were much lower than today.
Researchers believe they may be fish traps built on the foreshore or walls to protect against rising waters.
The study says the structures reflect “technical skills and social organization sufficient to extract, move and erect blocks weighing several tonnes, similar in mass to many Breton megaliths”, large arrangements of stone used as monuments or for ceremonial purposes.
This technical know-how would predate the first megalithic constructions by several centuries.
The study’s authors believe the site could be the origin of local Breton legends about sunken towns, according to BBC News. One of these lost cities – Ys – was thought to be in the Bay of Douarnenez, a few kilometers to the east.
“It is likely that the abandonment of a territory developed by a very structured society is deeply anchored in memories,” indicates the study. “The submersion caused by the rapid rise in sea levels, followed by the abandonment of fishing structures, protection structures and habitation sites, must have left a lasting impression.”
The discovery comes about six months after archaeologists discovered the remains of a 16th-century merchant ship. over 1.5 miles underwater off the south of France, the deepest discovery in the region.
