Ancient stone walls & dams have been discovered 9 metres underwater off the coast of Île de Sein in Brittany, France, dating to roughly 5950–3650 BCE & marking a key Mesolithic–Neolithic transition.

These granite structures likely served as large fish traps & possibly as protective sea walls, implying organised, semi-sedentary coastal communities long before many famous megaliths on land.

Research suggests that rising sea levels gradually drowned these installations & that later Breton legends of a drowned city may preserve a distant memory of this lost shoreline.

The tale of the sunken city of Ys is the most famous one, believed to lie a few kilometres east of the wall's location.

The undersea wall shows advanced stone-working skills being passed from older Mesolithic hunter-gatherers to incoming Neolithic farmers around 5500–5000 BCE.

Its monoliths are about 500 years older than land equivalents, implying shared techniques for quarrying, cutting, & moving stone across a shift from foraging to farming.