Monumental walls discovered on the seabed of Brittany: underwater archaeology reveals structures that defy the boundaries of history.

Amidst the rushing currents of the Iroise Sea, where the mainland seems to surrender to the ocean, the seabed reveals the traces of a past civilization who had learned to ask himself how to live with the advancing waters.

A short distance from theIle-de-Sein, where the English Channel meets the open ocean, some have been discovered monumental submerged structures which today lie nine meters deep. Perhaps they were protective dams, perhaps fish traps. In any case, the French researchers' discovery challenges our understanding of prehistoric societies and their connection to the sea.

Man-made walls 9 meters deep: the discovery

"We were returning from a dive near the Ar-Men lighthouse, where we were looking for cannons”, he told Le Monde Philippe Bodénès, president of the Society of Maritime Archaeology and Memory (SAMM). On the way back to theIle-de-Sein, continues the expert diver, “we dived again because there was still air in the tanks”. It was that dive, in May 2022, that revealed the existence of a underwater wall at a depth of 9 meters – the largest submerged structure ever discovered in France.

The discovery was not entirely accidental: studying a new map of the seabed off the coast of Finistère generated with LIDAR technology, the geologist Yves Fouquet He had just spotted a straight line, about 120 meters long, that seemed to cut cleanly across the underwater valley. A decidedly unnatural-looking line.

SAMM researchers then dived in that area between 2022 and 2024. About two kilometers away from the Île-de-Sein, in the direction of the open ocean, the divers located a large stone wall, 20 meters wide on average and two meters high, composed of about sixty granite blocks placed at regular intervals. This imposing structure, with an estimated total mass of over 3 tons, took the name of TAF1, an acronym derived from Toul ar Fot, the Breton name for this stretch of sea.

The investigations have revealed in all 11 submerged structures, at depths between 7 and 9 meters. According to the study published in theInternational Journal of Nautical Archeology, these are very ancient buildings:

“Based on sea level data, the dating of these structures has been estimated between 5800 and 5300 BC. These remains, unique at such a depth, show the presence of humans and advanced construction skills in the Mesolithic, predating Neolithic megalithism in Brittany by 500 years,”

we read in the research.

Fishing structures or barriers against the abyss?

The data on sea ​​level have been essential for reconstructing the history of these submerged structures. Between 8000 and 6000 BC, the researchers explain, in western Brittany the rate of rise was between 5 and 8,4 millimeters per year. This figure alone is enough to give an idea of ​​how rapid theexpansion of the seas: in recent years, those of the full-blown climate crisis, the rate has hovered between 2 and 4,5 millimeters.

The ocean has redrawn the map of the region, especially at the time when the submerged walls of the Île-de-Sein were built. As the study states,

“Around 5950 BC, the island was 7,5 kilometers long and up to 2,5 kilometers wide. The land area covered 10,2 square kilometers, 14 times its current surface area.”

Those walls, the researchers hypothesize, could be defensive dams which intended to protect coastal settlements from the advance of the sea or fish trapsThe construction of wooden or stone dams and barriers was in fact a very widespread fishing technique among the European populations of the Mesolithic, even if there is no evidence of such massive dams or ones built with the same technique.

In any case, these are monumental structures: their dimensions, together with the volumes of rock moved and the technical skills involved in the construction and maintenance of the artifacts, suggest “a strong social organization in a population that may have become sedentary”. And it is possible, the research continues, that “the abandonment of a developed territory by a highly structured society has become deeply rooted in people's memories".

The Legend of Ys: The Myth, the Science, and the Rock

The rapid rise in sea level and the consequent flight from the coasts, the study states, "must have left an indelible mark” in the collective memory. The reference is to the Legend of Ys, the mythical island built in the seas of Brittany by Gradlon, king of Cornwall. According to legend, the island was located below sea level and was protected by a dam that allowed ships to enter during low tide. The only key that opened the gate to this dam, legend has it, was in the possession of the king.

The city of Ys, often described as a crossroads of artistic fervor and mercantile exchange, ended up submerged when the keys came into the hands of Dahut, daughter of Gradlon (or, according to some versions, of her foreign lover).

This Breton legend could be the trace of an event that really happened in the Iroise Sea, and the myth of the city ​​lost beneath the waves It could be the same men who built the granite walls we see today for the first time. The findings of Toul ar Fot, the study states,

“they allow us to question the origins of the history of the city of Ys, not starting from historical legends and their numerous additions, but from scientific discoveries that could be at the origin of this legend.”

The hazy boundaries of the myth could thus be intertwined with scientific field observations and new knowledge about the rising sea levels in western Brittany. The detailed study of submerged landscapes, made possible by technologies such as LIDAR, thus offers a first-hand look at past civilizations erased by the seas, but whose legacy remains intact – in myth as in rock.

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