Divers have found a roughly 7,000-year-old stone wall in the sea off Île de Sein, on Brittany’s western coast in France. The wall is about 120 metres (394 feet) long, built from large granite slabs with upright monoliths at regular intervals, and lies 9 metres underwater. Dating to about 5800–5300 BC, it was built when sea levels were lower, with about a dozen smaller stone structures nearby. Researchers suggest it may have been a fish trap or a protective dyke against rising water. The video speculates that the site may be connected to the legendary lost city of Ys. Maps comparing 7,900 years ago with today show how much of the island’s former land area was drowned by rising seas.
7,000-year-old stone wall found off Île de Sein, Brittany
Site mentioned
📍 Île de Sein
Key facts
- The site is off Île de Sein, at the western tip of Brittany, France.
- A map compares the larger landmass around the island 7,900 years ago (black outline) with the modern coastline (white outline).
- The wall is about 120 metres (394 feet) long, lies 9 metres underwater and dates to roughly 5800–5300 BC.
- It contains large granite slabs and regularly spaced upright monoliths, with about a dozen smaller stone structures reported nearby.
- Researchers suggest the wall may have functioned as a fish trap or a protective dyke against rising water.
- The video speculates that the site may be connected to the legendary lost city of Ys.
Sources
the-sun.com
Linked in video description
popularmechanics.com
Linked in video description
Île de Seinen.wikipedia.org
Huge undersea wall dating from 5000 BC found in France". "Submerged Stone Structures in the Far West of Europe During the Mesolithic/Neolithic Transition (Sein ...
Huge undersea wall dating from 5000 BC found in Francebbc.com
The 120-metre (394ft) wall – the biggest underwater construction ever found in France – average 20 metres wide and two metres high.
Divers discover massive 7000-year-old undersea wall off ...cbsnews.com
Dating from between 5,800 and 5,300 BC, they lie nine meters underwater and were built at a time when sea levels were much lower than today.
Huge ancient undersea wall dating to 5800 BCE ...archaeologymag.com
Divers discover a massive 5800 BC undersea wall off the French coast, revealing advanced coastal society over 7000 years ago.
Submerged Stone Structures in the Far West of Europe ...hal.science
by Y Fouquet · 2025 · Cited by 2 — Morpho-tectonic analysis of LIDAR data off Sein Island (France) revealed 11 submerged structures at significant depths.
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