Ruins of a prehistoric stone wall submerged off the northwest coast of France may be evidence that a sunken city of legend actually existed.
The megalithic structure is thought to be about 7,000 years old, and could have been used either as a trap for fish or to protect a nearby settlement from flooding.
This find could mean that more submerged ruins could be in the region, ready to tell the story of a time when Mesolithic hunter-gatherer societies transitioned to Neolithic settlements.

https://www.popularmechanics.com/science/archaeology/a69723751/7000-year-old-sunken-city/