"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.

Researchers believe they may have been fish traps built on the foreshore, or walls to protect against rising seas.

The study says the structures reflect "technical skills and social organization sufficient to extract, move and erect blocks weighing several tonnes, similar in mass to many Breton megaliths", large stone arrangements used as monuments or for ceremonial purposes.

This technical know-how would predate the first megalithic constructions by several centuries."