Interesting topic, I've just started reading a book about the Celts (conveniently named "The Celts"

, by Jean Markale) & he's got a whole chapter about their ancient myths & how a very common theme is towns submerged in water or towns being destroyed by the sea, which he believes were legends passed down after groups of Celts expanded from their original homeland (in the center of modern-day Germany near Harz) to settle along the Baltic & North Seas coast- which, because at the time (around 1200 b.c) the climate was relatively dry & warm, was a fine place to inhabit- but by the end of the Halstatt period (around 530 b.c) Western Europe became suddenly very cold & wet again , causing huge areas of the coastline being flooded. As a result those Celtic people migrated away from their old homes & brought their story of the devastating floods with them, hence the legends that arose later. He even quotes Greek writers who came into contact with these myths, including the legend that Celts used to perform a 'water ritual' to drive back the waves- apparently by gathering arms & marching against the sea

!! As quoted in the book-
"Neither do i believe, as the historian had told us, that the Cimbri [Celts] brandish their weapons at the mounting waves to drive them back, nor, as Ephorus says of the Celts or Gauls, that they train themselves to fear nothing by calmly watching the sea destroy their homes, which they later rebuild, and that the floods have claimed more victims among them than war." (Strabo, VII, 2.)
There's more but yeah, think I've prattled on enough

, just thought that was an interesting example of a historical event becoming folklore, well at least from what I've read in a chapter of this book- can't be too sure of it's accuracy, & it
was written in 1976 so maybe more archaeological evidence was been discovered since then but i think the quote from Strabo is quite interesting regardless ...