Key discovery: A nearly 400-foot granite wall and about a dozen associated stone features were found off Île de Sein in Brittany, dated to roughly 5,800–5,300 BC and now lying nine meters underwater. Researchers suggest the alignments may have served as fish traps or defensive sea walls as coastlines shifted. The construction required moving multi-ton blocks—technology that could predate known megalithic building in the region—and may explain Breton legends of sunken cities like Ys.
7,000-Year-Old Granite Wall Found Underwater Off Brittany — May Explain ‘Sunken City’ Legends
Divers have discovered a long-submerged wall some 7,000 years old under the sea off western France, scientists said Thursday. / Credit: SAMM, 2023/Yves Fouquet et al. International Journal of Nautical Archaeology (2025)
Divers and marine archaeologists have documented a nearly 400-foot (about 120 m) granite wall and a cluster of smaller stone features submerged off the Île de Sein in western Brittany. The constructions have been dated to roughly 5,800–5,300 BC and now lie about nine meters beneath the sea surface.
Discovery and Investigation
The features were first identified in 2017 by retired geologist Yves Fouquet after they appeared on seafloor charts created with a laser-mapping system. Diving teams surveyed the site between 2022 and 2024 and confirmed the presence of large, well-preserved granite blocks arranged in a long linear alignment and a dozen nearby features.
"This is a very interesting discovery that opens up new prospects for underwater archaeology, helping us better understand how coastal societies were organized," said Yvan Pailler, professor of archaeology at the University of Western Brittany and a co-author of the study published in the International Journal of Nautical Archaeology.
Age, Purpose and Significance
Radiocarbon and contextual evidence date the constructions to between about 5,800 and 5,300 BC — a time when sea levels were substantially lower than today. Researchers propose the alignments could have functioned as fish traps on the foreshore or as protective walls to buffer shorelines from rising waters during a period of rapid sea-level change after the last Ice Age.
The study highlights the technical skill required to extract, move and erect multi-ton granite blocks, noting their mass is comparable to many later Breton megaliths. If confirmed, that engineering capability would precede the region's earliest known megalithic monuments by several centuries.
Legend, Memory and Broader Context
The authors suggest the submerged complex may lie behind local Breton tales of drowned cities, including the legend of Ys in the Bay of Douarnenez a few miles to the east. The study argues that rapid submersion, abandonment of fishing structures and other coastal installations could have left a lasting cultural memory preserved in oral tradition.
As a point of regional context, the discovery follows other recent underwater finds in French waters, including the remains of a 16th-century merchant ship located more than 1.5 miles underwater off southern France.
Implications
The site illustrates the potential for well-preserved prehistoric coastal remains on the modern seafloor and underscores how post-glacial sea-level rise reshaped human settlement and activity. Researchers say further survey and excavation will be needed to confirm the functions of the structures and to better understand the people who built them.
