A massive underwater stone wall that is almost 7,000 years old is found by divers.

One of the buried stone monuments on Sein Island is a granite wall off Brittany that is more than 7,000 years old and the length of a football pitch. The discovery is being used by marine archaeologists in the vicinity of Île de Sein to explain how stone-built coastal villages predate the widespread spread of farming.

LIDAR, a laser scanning technique that uses light pulses to determine distance, was utilised in laser surveys, and the lines were too straight for reefs. Yves Fouquet, a geologist with the Maritime Archaeology and Heritage Society, oversaw the project.

Divers assist in converting maps into measured stone designs, and his research focuses on underwater remnants of ancient beaches. Using LIDAR maps to direct dives towards the clearest structures, SAMM divers returned to the site to study a submerged stone wall. They recorded every slab without moving it using photogrammetry, a program that creates 3D models from images. A 3.3-foot rod next to an upright stone in one picture provides scale for the current depth.

TAF1 is a 394-foot-long wall made of granite blocks that spans a drowned valley west of Sein. The summit is reinforced by over 60 monoliths, which are slabs that rise to a height of almost 6.6 feet. A single stone is fixed upright in place.

Although this strengthening implies purposeful engineering, the wall's original coastline is disputed because it is located many yards offshore.

The technique is shared by other builds.

TAF2A, TAF2B, and TAF3 are nearby constructions that use the same combination of uprights and stacked blocks. A rope placed on the summit in the pictures indicated two parallel rows that were roughly 4.9 feet apart. Repeating the pattern in different structures suggests that there is common knowledge rather than a single group's experiment.

2024 dives also discovered narrower walls composed of smaller stones that were positioned to obstruct bedrock's natural dips. One line, known as YAG3C, is 164 feet long and consists of tiny monoliths arranged in parallel rows near to one another.

Because tiny stones can fill in spaces where water is drawn in by the tides, these tighter constructions suggest a different objective.

When traps are created by walls

Stone was frequently utilised by coastal builders to direct fish, transforming a straightforward barrier into a reliable harvesting technique. When gaps remain low and narrow, a fish weir—a fence-like trap that captures fish when tides drop—works well.

If a number of Sein buildings were traps, their builders probably knew when the tides were coming and kept stone lines intact for many years.

Sea level change as a dating method

The team used local sea-level histories and bottom depth to determine age because there was no wood or bone. They located 11 structures between 5800 and 5300 BCE using relative sea level and coastline height in relation to land at that period.

Future dating will rely on locating dateable material in the area because that method provides a range rather than a precise year.

The meaning of the Mesolithic

Before farming became popular in Europe, the date period corresponds to Mesolithic and late Stone Age life. Although people continued to hunt, fish, and harvest wild herbs, living by the coast required preparation due to the rigid schedules set by storms and tides.

Archaeologists are forced to reconsider which skills emerged just after crops and livestock when they discover substantial stone construction during this time.

Who constructed the wall of submerged stone?

Because builders had to coordinate timing, equipment, and safety on a tidal plain, extracting and transporting granite blocks required coordinated routines. Teams most likely employed hauling lines and leverage, which alters what a small group can exactly lift and put.

Even though the inhabitants left few permanent houses on land, such cooperation indicates societal norms and leaders. The largest wall might have indicated a border or buffered waves, but other lesser structures resemble traps.

By reducing the amount of sand that is carried away by flowing water, a barrier across a valley can lessen erosion. Since there are now no artefacts next to the stones, function is still a theory that needs to be supported by additional data.

Why divers were taken aback by preservation

Divers can find bare stone in the winter and covered stone in the summer due to strong currents and cold water shape visibility. According to Fouquet, "archaeologists did not anticipate finding such well-preserved structures in such a harsh setting."

Although the same water movement can restrict dive time and make thorough sampling more difficult, clear surfaces aid in mapping. For a long time, Brittany has heard tales of a drowned city close to Sein, by the Bay of Douarnenez.

Camps and routes may be destroyed by rising waters, and tales of retreating land may endure for many centuries as folklore. Legends cannot be directly tested by archaeology, but the discovery demonstrates that actual locations did vanish as the coast altered.

This buried stone wall teaches us

In an effort to identify more buildings and any indications of habitation, researchers intend to conduct additional LIDAR mapping in the Sein area.

Sea levels rise as a result of melting glaciers and warmer ocean water. Because storms can disrupt shallow seabed locations and every lost shoreline conceals human footprints, this trend heightens urgency.

When considered collectively, the buried stone constructions on Sein Island demonstrate how coastal people managed large-scale construction while still primarily depending on natural resources.

Wider surveys and improved dating could clarify if the work was driven by social marking, fishing, or shoreline defence, as well as how frequently it occurred.