Scientists have discovered the underwater ruins of huge stone structures erected by humans at least 7,000 years ago in the coastal waters of France, according to a new study published in the International Journal of Nautical Archaeology.The submerged granite ruins near Sein Island, a Breton island in the Atlantic Ocean, are among the oldest large stone structures ever found in France, and may have inspired an ancient local legend about a city called Ys that vanished under the waves.The structures vary in size from small stone dams, which were probably fish traps, to large monoliths and slabs that protrude six feet from the seafloor and extend 400 feet in length, which perhaps once served as a protective seawall.Yves Fouquet, a geologist who works with the Society for Maritime Archaeology and Heritage (SAMM), first noticed hints of these long-lost megaliths in LiDAR data collected by the Litto3D program, a national initiative to create a precise 3D digital reconstruction of the entire French coastline. Fouquet and his colleagues confirmed the existence of the mysterious structures, and mapped out their locations, across dozens of dives carried out by ten SAMM divers between 2022 and 2024.The detailed analysis of these maps to redraw the underwater geological map of this area (faults, rock types) has made it possible to identify structures that did not appear natural to a geologist, Fouquet said in an email to 404 Media.Brittany, a peninsular region of northwest France, is home to the oldest megaliths in the nation and some of the earliest in Europe, which date back some 6,500 years. The team estimated that the submerged stone structures off Sein Island may predate these early megaliths in Brittany by about 500 years, based on their estimation of when the stones would have last been above sea level. But it will take more research to home in on the exact age of the megaliths.We plan to continue the exploration and carry out more detailed work to understand the architecture and precise the age of the structures, Fouquet said. The discovery of these stones opens a new window into the societies living in Brittany during the Mesolithic/Neolithic Transition, a period when hunter-gatherers began to shift toward settled lifestyles involving fishing, farming, and the construction of megaliths and other buildings.Photos of the structures in Figure 7 of the study. Image: SAMM, 2023The peoples who made these structures must have been both highly organized and relatively abundant in population in order to erect the stones. They were also sophisticated marine navigators, as the waters around Sein Island are notoriously dangerousprone to swells and strong currentswhich is one reason its underwater heritage has remained relatively poorly explored.Our results bear witness to the possible sedentary lifestyle of maritime hunter-gatherers on the coast of the extreme west of France from the 6th millennium onwards, said Fouquet and his colleagues in the study. The technical know-how to extract, transport, and erect monoliths and large slabs during the Mesolithic/Neolithic transition precedes by about 500 years the megalithic constructions in western France in the 5th millennium.The discovery raises new questions about the origins of these megalithics structures, which may have had a symbolic or religious resonance to these past peoples. the team added. This discovery in a high hydrodynamic environment opens up new perspectives for searching for traces of human settlement in Brittany along the submerged coastline of the period 60005000 years cal. BCE.The researchers also speculate about a possible link between these structures, and the prehistoric people who made them, and local legends about sunken cities that may date back thousands of years.Legends about sunken cities, compared with recent data on rising sea levels, shows that the stories of ancient submergences, passed down by oral tradition, could date back as far as 5,000 to 15,000 years, the team said, citing a 2022 study. This suggests that oral traditions that may have preserved significant events in memory that could well be worthy of scientific examination. These settlements described in legend reveal the profound symbolic significance of maritime prehistory, which should not be overlooked.In particular, the people of Brittany have long told tales of the lost city of Ys, a sunken settlement thought to be located in the Bay of Douarnenez, about six miles east of Sein Island. The sunken megaliths off Sein Island allow us to question the origin of the history of the city of Ys, not from the historical legends and their numerous additions, but from scientific findings that may be at the origin of this legend, the team said.Its extremely tantalizing to imagine that the long-hidden ruins of these peoples, who appear to have been expert seafarers and builders, are the source of tales that date back for untold generations in the region. But while the researchers raise the possibility of a link between the stones and the story, they cannot conclusively confirm the connection.Legend is legend, enriched by all the additions of human imagination over the centuries, Fouquet said in his email. Our discoveries are based on what can be scientifically proven.Subscribe to 404 Media to get The Abstract, our newsletter about the most exciting and mind-boggling science news and studies of the week.