Breton fishermen have long told of a drowned city beneath the waves near Île de Sein: the city of Ys, built below sea level behind dikes and sluice gates, lost when the gates were opened and the Atlantic poured in. In 2022, the legend gained an unexpected archaeological echo — divers touching a 120-meter granite wall on the seabed, 1.9 km west of the island.
How the discovery began
French geologist Yves Fouquet noticed unnatural linear structures in Litto3D LiDAR bathymetry data near Sein Island. The lines crossed valleys obliquely to geological faults, repeating across adjacent valleys — patterns natural erosion cannot produce. The Société Académique de Marine Marchande (SAMM) divers began surveying in summer 2022; after winter kelp receded in 2023, they confirmed the walls. Over three years, 10 divers made 59 dives and documented 11 artificial granite structures on the seabed.
The wall: structure and design
The largest structure, TAF1, is 120 m long and averages 20.9 m in width — five times wider than any known Breton fish weir. Divers identified layered construction:
1. Vertically planted monoliths — 62 granite stones in two parallel rows ~1.5 m apart, cross-sections ~0.5×0.3 m with straight edges, estimated 3 m tall (1.7 m exposed), anchored 1.3 m deep and systematically quarried from nearby reefs. 2. Large slabs inserted vertically between monoliths, forming continuous panels. 3. Angular rubble fill interlocking between monoliths and slabs for stability. 4. Small slab facing on the top and sides, scaled like armor against surge scour.
Notably, the wall is asymmetric: the north side, facing Atlantic swell, is much wider — deliberate wave-damping, not weathering. On the Ar Fot Blad reef, just 100 m from TAF1's western end, divers found a sharp-edged rectangular pit (25×10 m, 7 m deep) with no fallen debris inside — likely the quarry where the monoliths and slabs were cut.
What was it for?
Two hypotheses, both pointing to the same startling conclusion:
- Fish weir: Breton weirs should not exceed 16.7% of the local tidal range. With Sein's 6.81 m tidal range, the height limit is ~1.13 m — but TAF1 stands 1.7 m, 50% over.
- Protective sea wall: This explains the oversized height (storm defense), asymmetric profile (wave dissipation), and deeply anchored monoliths (storm survival). Behind the wall lies an enclosed shallow basin — possibly dwellings, a harbor, or both.
- Memory: A real event survived nearly 10,000 years in oral tradition, distorted in detail but faithful in core: walls by the sea were drowned by the sea. Flood myths may be memories, not metaphors.
- Overlooked evidence: 11 structures sat 7,800 years less than 2 km from an inhabited island, noticed only once someone examined the seabed map.
- Engineering continuity: The builders selected quarries by geological jointing, built layered storm-resistant structures, optimized wave dissipation with asymmetric cross-sections, and used angular rubble for internal friction. The gap between them and us lies in accumulated knowledge, not intelligence.
Either way, construction dates to 5800–5300 BCE, the Mesolithic-Neolithic transition, when Brittany's inhabitants were still maritime hunter-gatherers. TAF1's standing monoliths predate Brittany's earliest menhirs (~5300 BCE) by roughly 500 years — potentially the true origin of Europe's megalithic tradition.
Echoes of the myth
Between 8000 and 4500 BCE, relative sea level rose ~25 m along this coast. Coastal peoples watched the sea swallow their ancestors' walls for generations; the legend of Ys — a walled settlement drowned by the sea — plausibly preserves, in mythologized form, a real memory of drowned coastal engineering. The fishermen were right: there are walls under the sea.
A drowned world
Sein is not alone. In 2024, German researchers reported the Blinkerwall in the Baltic's Bay of Mecklenburg — a 971 m wall of over 1,300 stones used to herd reindeer. In 2014, a US team found the 9,000-year-old Drop 45 Drive Lane hunting complex beneath Lake Huron. Since the last Ice Age, sea levels rose ~120 m, drowning the coastlines where humans lived for tens of millennia. Our picture of prehistory is biased toward upland and inland sites — not because lowlands were empty, but because they are underwater. Sein's granite seabed preserved its structures; sandy coasts likely preserved nothing. Global high-resolution bathymetric scanning of shallow coastal waters could reveal many more "cities of Ys."
Takeaways
*Reference: Fouquet, Y. et al. (2025). Submerged Stone Structures in the Far West of Europe During the Mesolithic/Neolithic Transition (Sein Island, Brittany, France). International Journal of Nautical Archaeology, In Press.*
