Marine researchers have uncovered a vast prehistoric stone structure lying beneath the waters off the western coast of Brittany, France-an extraordinary discovery that is reshaping understanding of early coastal societies in Europe.
The underwater structure, located near the Île de Sein, consists of a massive stone wall stretching roughly 120 meters across a submerged valley. Measuring about 20 meters wide and rising nearly two meters from the seabed, the wall is estimated to weigh more than 3,300 tonnes. Archaeologists say it is the largest ancient underwater construction ever documented in France.
Alongside the main wall, scientists have identified multiple smaller stone features, including aligned monoliths arranged in long rows. Together, the structures suggest a carefully planned project rather than a natural formation, pointing to a high level of organization among the people who built it.
Discovered Through Advanced Seabed Mapping
The site was first noticed in 2017 during a detailed review of seabed maps created using laser and radar imaging technologies. The unusual linear shape stood out clearly against the surrounding terrain, prompting further investigation. Between 2022 and 2024, teams of marine archaeologists conducted diving expeditions to examine the area directly, confirming the artificial nature of the structures and documenting their layout in detail.
Researchers were surprised by the preservation of the wall, given the strong currents and harsh conditions typical of the Atlantic coastline.
Built When the Sea Was Lower
Based on the structure’s depth-about nine meters below today’s sea level-scientists estimate it was constructed between roughly 5,800 and 5,300 BCE. At that time, sea levels were significantly lower, and the site would have been located along the shoreline rather than underwater.
This places the construction during the transition from the Mesolithic to the Neolithic period, centuries before Brittany’s famous standing stones and stone circles were erected on land. The finding challenges long-held assumptions about when large-scale stone construction began in the region.
Possible Uses: Fishing or Coastal Defense
Archaeologists believe the wall may have served as an advanced fish-trapping system. The upright stones could have supported wooden barriers or nets, allowing fish to be captured as tides moved in and out. Another theory suggests it functioned as a protective barrier, helping shield coastal settlements from encroaching seawater during a time of rising sea levels.
Regardless of its exact purpose, experts agree the structure required coordinated labor, technical knowledge, and long-term planning-evidence of a settled or semi-settled community capable of large collective projects.
A Society Lost to the Sea
The scale of the construction indicates the builders were not small, scattered groups, but a well-organized society. Researchers suggest they may have been late hunter-gatherers transitioning to a more sedentary lifestyle, or some of the earliest farming communities moving into the area. There may also have been an exchange of knowledge between indigenous populations and incoming Neolithic groups.
Echoes of Ancient Legend
Intriguingly, scientists note that the discovery lies close to areas associated with Breton legends of submerged cities, particularly the mythical city of Ys. While the legend is not taken literally, researchers believe real memories of land lost to the sea may have been passed down through generations, eventually evolving into folklore.
A Window Into Europe’s Drowned Past
The find highlights how much of Europe’s early human history may now lie underwater, submerged by post-Ice Age sea-level rise. Further research is planned to refine the dating of the site, study construction techniques in greater detail, and search for additional traces of ancient habitation along the now-drowned coastline.
Experts say the discovery offers rare insight into how early communities adapted to environmental change-an issue that remains highly relevant today.
