The submerged remains of a megalithic wall discovered off the Île de Sein in Brittany may be a 7,000-year-old human-built structure, dated to about 5,800–5,300 BCE. LIDAR and relative sea-level analysis reveal eleven features weighing roughly 3,300 metric tons and include monoliths nearly ten feet high. Researchers propose the alignment functioned as either a large fish weir or a protective dyke, implying organized labor and complex engineering. The find could help explain local legends such as the drowned City of Ys and point to more submerged prehistoric sites.
7,000-Year-Old Sunken Wall Discovered Off Brittany — Ancient Megalith May Explain Legend of Ys
Archaeologists Found Evidence of a Sunken CityGiordano Cipriani - Getty Images
Marine archaeologists and geologists have identified the submerged remains of a prehistoric stone alignment off the Île de Sein, Brittany, that may represent a 7,000-year-old human-built structure. LIDAR and relative sea-level analysis date the features to roughly 5,800–5,300 BCE and suggest they were engineered either as a large fish trap or as a protective dyke for coastal settlement areas threatened by rising seas.
How the Structure Was Found
Geologist Yves Fouquet first noticed a straight, artificial-looking line on undersea depth charts derived from radar. Subsequent surveys using LIDAR revealed eleven discrete subsea features that exhibit shapes and alignments unlikely to be natural. The array lies about 30 feet (approximately 9 meters) below the modern sea surface near an undersea valley close to Île de Sein.
Dating, Size and Construction
Relative sea level records, which account for both sea-level change and vertical land movement, place the structures between about 5,800 and 5,300 BCE — a period when coastal shorelines were several miles seaward of their current positions and when human groups transitioned from Mesolithic hunter-gatherer lifestyles toward more settled Neolithic communities. The identified stone fragments total roughly 3,300 metric tons, and the site includes standing monoliths nearly ten feet tall. The arrangement of slabs, blocks and boulders suggests multiple phases of construction and maintenance, with heavy, deeply anchored stones intended to resist strong hydrodynamic forces and storms.
Function and Cultural Implications
Researchers propose two main functions: a large-scale fish weir designed to channel and trap fish as tides fell, or a dyke-like barrier built to protect a shoreline settlement from flooding. Either interpretation implies considerable technical knowledge and social organization, because quarrying, transporting and erecting massive stones would have required coordinated labor and planning.
Connection To Local Legend
The discovery renews interest in regional myths such as the Breton legend of the drowned City of Ys, traditionally placed in the Bay of Douarnenez east of Île de Sein. While legends accumulate layers over centuries, Fouquet and colleagues argue that such scientific finds can prompt fresh questions about whether prehistoric events inspired later tales.
Fouquet and his team note that the difficult conditions around deep coastal zones — strong tidal currents, high hydrodynamic activity and seaweed cover — have left these areas understudied, and that low-resolution nautical charts previously concealed such archaeological evidence.
What Comes Next
Fieldwork in this high-energy coastal environment remains challenging, but targeted underwater surveys and excavations could reveal more about construction methods, the site's exact function and any associated artifacts or organic remains. If confirmed as human-built and associated with habitation, the wall would shed light on the Mesolithic-to-Neolithic transition along Atlantic Europe and could point to additional submerged sites awaiting discovery.
Source: Research published in the International Journal of Nautical Archaeology and surveys led by Yves Fouquet.
