what makes most sense to me, at least to explain stories in europe, is simply when doggerland got covered by the ocean. That’s a fucking big landmass that would have been prime real estate, no shit it being lost to water would be something people make stories about. And back then people didn’t live their entire lives in one village, they migrated all the time and people would hop between tribes and shit, lots of people would have personally seen that it was now underwater and spread that knowledge around.