7,000-year-old secret: a remarkable underwater wall discovered off the coast of France

Off the coast of Brittany, researchers have uncovered a monumental underwater wall which, more than seven thousand years ago, was still part of the mainland. The find sheds new light on the lives of ancient communities and their adaptation to their environment.

French archaeologists have made an extraordinary discovery off the western coast of Brittany: on the seabed they found the remains of a vast stone wall, approximately 120 metres long, which according to current knowledge is the largest underwater wall in France.

Researchers believe the wall was built between 5800 and 5300 BC, more than seven thousand years ago, at a time when the coastline of the region differed radically from today, reports the BBC.

The underwater wall was once part of the coast

The structure was found near the Île de Sein, at the westernmost point of Brittany. When the wall was constructed, it lay along the coastline, in the zone between low and high tide. Due to rising sea levels, much of the area has since been submerged, meaning that today the wall lies at a depth of around nine metres on the seabed, while the island itself has shrunk to only a fraction of its former size.

The dimensions of the structure are impressive: it is on average 20 metres wide, around 2 metres high, and is estimated to weigh approximately 3,300 tonnes. These figures clearly indicate that the construction of the wall required a high degree of organisation and long-term planning.

A fish trap or a defensive barrier?

Archaeologists have proposed two different theories regarding the function of the wall. One idea is that it served as a fish trap, as large granite monoliths were found along the wall, rising from the structure in two parallel lines.

According to the researchers, these standing stones may have supported a mesh made of poles and branches, which would have trapped fish moving with the retreating water at low tide.

Another possible function of the underwater wall may have been flood defence, as it is also conceivable that the wall was built to protect against gradually rising sea levels. The two functions do not necessarily exclude one another, since the structure could have served both as a means of subsistence and as protection for inhabited areas.

Who might have built the wall?

According to archaeologist Yvan Pailler, the underwater wall was either created by hunter-gatherer communities who settled here due to abundant natural resources, or by early Neolithic peoples who arrived in the region between 5500 and 5000 BC.

The monoliths forming the basis of the underwater wall are particularly noteworthy, as they predate the famous menhirs that later became widespread in Brittany. This suggests that techniques of stone extraction and working may already have been passed down among Mesolithic and Neolithic communities.

Discovery through modern technology

The existence of the wall was first noticed by Fouquet while studying radar-based underwater bathymetric maps. On these, an unnatural, 120-metre-long line appeared, blocking an underwater valley. The first archaeological dives took place in the summer of 2022, but detailed mapping could only be carried out the following winter, when algae and marine vegetation had retreated sufficiently.

In the footsteps of legends

According to the researchers, the discovery goes beyond architectural and archaeological significance. They suggest that such submerged structures may have inspired the legends of sunken cities widespread around Brittany, including the famous story of the city of Ys, which tradition holds was swallowed by the sea in the Bay of Douarnenez.

During the period between 8000 and 4500 BC, significant sea-level rise may have submerged numerous settlements. The sight of abandoned fishing and defensive structures, as well as former dwellings, could easily have left a lasting impression on later generations.

• Sunken city and Islamic necropolis discovered by archaeologists!
• Unexpected discovery: Second World War shipwreck found in the depths of the Pacific Ocean after 80 years