Under several meters of water, off the coast of Brittany, a 120 meter long stone structure challenges our understanding of prehistoric coastal societies. Spotted thanks to LIDAR surveys and confirmed by dives between 2022 and 2024, this massive construction dates from 5800 to 5300 BCE, well before the famous Breton megalithic alignments. The study, signed by researchers from the CNRS, the University of Western Brittany and Nantes University, was published in The International Journal of Nautical Archaeology.
It reveals the existence of elaborate technical know-how and a structured social organization among Mesolithic groups, long perceived as nomadic and not very hierarchical. This discovery raises new hypotheses on the occupation of the coastline at the end of Prehistory, and invites us to reconsider the place of these societies in the history of monumental constructions in Europe.
Monumental architecture under the sea revealed by technology
The main structure, identified as TAF1, is 120 meters long, approximately 20 meters wide and up to 2 meters high. It lies at a depth between 7 and 9 meters below current sea level, at the western end of Brittany, near the island of Sein. A morpho-tectonic analysis by bathymetric LIDAR, a high-resolution underwater laser mapping system, used in 2017 by geologist Yves Fouquet, researcher at the CNRS, revealed its existence. The latter detected a series of linear anomalies which did not correspond to any natural formation.
Dives carried out between 2022 and 2024 allowed the confirmation of the discovery. The structures feature complex assemblages of granite blocks, associated with upright monoliths anchored in the bedrock. These vertical stones are arranged in a double alignment, spaced approximately 1.5 meters apart, and embedded in a filling of angular slabs and blocks.
According to researchers, the thoughtful layout and remarkable stability of the building – despite more than seven millennia of marine erosion – testify to mastered architectural know-how. No other structure of this scale and period is documented in France.
The 3D mapping of the site highlights the regularity of the construction, and an asymmetrical profile, reinforced on the Atlantic side. These characteristics indicate a clear intention of sustainability in a dynamic environment. The wall corresponds to a real infrastructure built with anticipation and method.
Multiple functions of a unique prehistoric construction
The exact function of this structure remains subject to debate. But several hypotheses emerge from the clues found on site. The most widely used is that of fish traps, exploiting tidal cycles. We know this principle well in other prehistoric regions, notably in the Baltic Sea. However, the dimensions of the Sein wall make it a unique case.
In this interpretation, the vertical monoliths would have served as supports for nets, wooden racks or hedges of branches intended to retain fish during the ebb. The fact that the structure was initially built in the foreshore — the area between high and low tides — supports this reading. Yvan Pailler, archaeologist at the University of Western Brittany, explains this to AFP. “ The regularity of the alignment and the deep anchoring of the monoliths argue for a controlled use of the tides “.
© Yves Fouquet et al., 2025
Winter 2023 view of the TAF1 wall without algae, revealing the alignment of the monoliths and slabs.
But other avenues are also being explored. The volume of 3,300 tonnes suggests a function beyond fishing, possibly defensive, territorial or symbolic according to the researchers. This suggests secondary or complementary functions. The most plausible remains that of a coastal protection work. A structure capable of attenuating waves or currents in certain sensitive areas of the then emerged coast.
Researchers do not exclude symbolic or territorial uses either. Indeed, the construction of such a structured wall could also respond to spatial marking issues. Or structuring a living space, in a context of shared marine resources. The variations in the architecture of the eleven structures identified on the underwater plateau support the idea of a site with diversified uses, both economic, social and perhaps ritual.
A technical feat attributed to the last sedentary hunter-gatherers
The dating of the wall and associated structures remains a crucial point. It is based on the analysis of the sea level relating to the period considered. By combining geological data and paleo-environmental benchmarks, researchers estimate the construction period between 5800 and 5300 BCE. This chronological framework corresponds to the transition between the Mesolithic and the Neolithic in this part of Europe.
Contrary to popular belief, the builders were not yet farmers. Archaeological evidence argues for a hunter-gatherer society that was already partially settled. Probably, a society permanently established on a coastal strip that is now submerged, as Yvan Pailler points out. “ This is a company with sufficient organization to mobilize a large workforce, with precise technical skills. “.
© Yves Fouquet et al., 2025
3D view facing east of the TAF1 wall showing its linearity and the asymmetry of its sides.
The extraction and implementation of blocks weighing several tonnes requires a well-defined operational chain: quarry identification, size, transport, anchoring. These gestures are reminiscent of those observed in megalithic cultures. However, here they predate the first menhirs of Carnac or Locmariaquer by several centuries. Hence the hypothesis of a transmission of know-how between cultures, Mesolithic and Neolithic.
The technical rigor and durability of the building also indicate a long-term relationship, rarely associated with pre-agricultural populations. The idea that hunter-gatherers lived only in the moment or in permanent mobility no longer holds up against this data. On the contrary, these communities anticipated coastal dynamics and invested massively in collective infrastructure.
Sunken memory and link with the legends of Brittany
This type of construction resonates deeply with elements of collective memory. The published study evokes a possible correspondence between this submerged site and Breton legends. And in particular that of the town of Ys, submerged in the bay of Douarnenez, just a few kilometers from the site.
According to the authors, it may be plausible that the progressive submergence of a structured territory, used intensively for fishing and housing, left lasting traces in oral culture. “ The forced withdrawal of a developed space, following rising waters, may have produced stories of loss and disappearance », they note.
Researchers point to a known phenomenon: the transformation of collective memories into mythological stories, over generations. Here, the evolution of the Breton coastline saw hundreds of square kilometers of coastal land disappear between 6000 and 5000 BCE. This could have fueled these stories of submerged cities. This link between archaeological facts and oral traditions is rarely so well illustrated.
Beyond the myth, this progressive loss of territory questions the resilience of societies of the time. The Sein wall could thus mark an attempt to adapt, or even resist, the inexorable rise in water levels. It testifies not only to an early environmental awareness, but also to a lived experience of the climate crisis on a prehistoric scale. A memory today revealed by the stones, but long inscribed in the imagination.
Source: Yves Fouquet, et al., “Submerged Stone Structures in the Far West of Europe During the Mesolithic/Neolithic Transition (Sein Island, Brittany, France)”. International Journal of Nautical ArcheologyIn press
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