Off the western coast of France, archaeologists have discovered a sunken granite Mesolithic wall nine meters below sea level, which evidence suggests was constructed close to 7,000 years ago.
The 120-meter-long structure is one of several prehistoric artificial features of its era found in the region, with other similar structures known to exist nearby. The discovery, made near Île de Sein in Brittany, was recently detailed in a study published in the International Journal of Nautical Archaeology.
Underwater Archaeology
The underwater structure was spotted in 2017 by the study’s lead author, Yves Fouquet, who was examining laser-produced maps of the ocean floor. Follow-up research using LIDAR and underwater surveys confirmed that the strange features on the seafloor map were indeed manmade, rather than merely being unusual natural features.
Initially, archaeologists expected to find little because they believed the harsh underwater environment would have eroded the structures’ surfaces. However, dives conducted between 2022 and 2024 revealed the existence of remarkably well-preserved ancient materials.
Dating techniques place the site’s construction between 5,800 and 5,300 BCE. At that time, water levels in the region were much lower than they are today; during the Late Mesolithic and Early Neolithic transition, the area where the wall now sits would have been coastal land.
The researchers note that their work represents a significant achievement in underwater archaeology in the region and helps to advance our understanding of ancient European coastal societies.
Interpreting a Mesolithic Site
The wall is constructed of granite blocks, reinforced by large slabs and megaliths nearly two meters tall. Based on site analysis, archaeologists suggest that the megaliths were first set into the bedrock, with the remainder of the wall filled in around them.
Other structures co-located with the wall were produced in much the same manner, although one outlier had been a series of narrow walls featuring smaller stones, which block a natural depression in the terrain. Among these smaller stone features was a 50-meter-long string of tightly-packed monoliths occurring in parallel rows.
The team breaks the features at the site down into two main categories, interpreting the smaller parallel rows of stones as having been fish traps, while the larger walls were either used to hold back rising sea levels or to serve as territorial boundary markers.
Intriguingly, the researchers note that the work involved in the construction of the wall would have required the ability to move stone blocks weighing several tons in some instances. While other stone megalithic sites in the area feature blocks of similar size, their construction would have occurred centuries later, making this the earliest known example of such stone work in the region.
Connections to Myth and Prehistory
Recent underwater archaeological research has uncovered additional evidence of European coastal settlements that utilized advanced stone work well before land-based agriculture spread across the continent.
In the case of the recent discoveries at Île de Sein, archaeologists suggest that the features likely result from a group of hunter-gatherers who became sedentary thanks to the ongoing supply of food and other resources their proximity to the coast would have offered. Additionally, it is theorized that these ancient people may have passed their knowledge of stonework on to later agricultural settlers.
Also of intrigue is the fact that local legends bear some similarities to the recent discovery. Specifically, a legend from around Brittany holds that west of the Bay of Douarnenez, a drowned city known as Ys was believed to exist at the bottom of the ocean.
While archaeologists find no evidence that the structures are the remains of an urban center, and instead interpret them as coastal features, they are close to the same area where the legend places the drowned city. The team notes that while their discovery is not an exact match for traditional beliefs associated with the region, changes made to oral traditions passed down over many generations could potentially account for the discrepancies, suggesting that a connection may exist nonetheless.
Further work is scheduled at the site in the months ahead, which the team believes will help add to archaeologists’ growing knowledge of the ways coastal groups in ancient Europe developed in prehistoric times.
The paper, “Submerged Stone Structures in the Far West of Europe During the Mesolithic/Neolithic Transition (Sein Island, Brittany, France),” appeared in the International Journal of Nautical Archaeology on December 9, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.