Huge undersea wall dating from 5000 BC found in France
https://www.bbc.com/news/articles/crk7lg1j146o
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> With an overall mass of 3,300 tonnes, the wall must have been the work of a substantial settled community. And to have lasted 7,000 years, it was clearly an extremely solid structure.
> "It was built by a very structured society of hunter-gatherers, of a kind that became sedentary when resources permitted. That or it was made by one of the Neolithic populations that arrived here around 5,000 BC," said archaeologist Yvan Pailler.