According to most if not all Atheists (I'm excluding Epicure!), the Flood never happened.

If this is true, all human populations started out without a memory of a global Flood.

True for whatever became the Hebrews, whatever became the Greeks, whatever became the Babylonians (both Akkadian and Sumerian ethnics), whatever became the Hindus. And I am conservative, one could argue that Chinese, Norse, even Egyptians, had memories of the Flood too.

For some reason, a population shifted from having no memory at all of a global Flood, to believing one were survivors of a global Flood, through a very narrow bottleneck of one man and close kin. Let's check the Hindu version, of which I am less sure, here is wiki:*

The current universe, is asserted to be ruled by the 7th Manu named Vaivasvata.[2] Vaivasvata was the king of Dravida before the great flood.[4] He was warned of the flood by the Matsya (fish) avatar of Vishnu, and built a boat that carried the Vedas, Manu's family and the seven sages to safety, helped by Matsya. The tale is repeated with variations in other texts, including the Mahabharata and a few other Puranas. It is similar to other floods such as those associated with Gilgamesh and Noah.[5]

2) Roshen Dalal (2010). The Religions of India: A Concise Guide to Nine Major Faiths. Penguin Books. p. 229. ISBN 978-0-14-341517-6.
4) Alain Daniélou (11 February 2003). A Brief History of India. Inner Traditions / Bear & Co. pp. 19–. ISBN 978-1-59477-794-3.
5) Klaus K. Klostermaier (5 July 2007). A Survey of Hinduism: Third Edition. SUNY Press. p. 97. ISBN 978-0-7914-7082-4.

So, yes, Hinduism also says not only there was a Flood, but also a kind of Ark with very few human passangers.

But not only that. For Hindus and Greeks, you could rationalise to "Indo-Europeans" and for Hebrews and Akkadians you could rationalise to "Semites" - but that still leaves three distinct ethnic groups without a common ancestor. "Indo-Europeans", "Semites" and Sumerians. This means, at some point, group A met group B, and group A had no memory of surviving a global Flood, group B had, and for some unfathomable reason, instead of saying "you know what, as far as we can recall, we don't descend from you, so we don't descend from that man on a boat, so he can't have been the sole survivor of a global Flood!" instead of that they go "is that so? how come we forgot it?" - How did they get to gaslight themselves that much?

I consider that idea of how the "Flood myths" started very incredible.

Now, a certain number of people 100 years ago would nearly automatically reply "but look, the Greeks for some reason came to believe in Hercules, and we know that stuff didn't happen!" - it so happens, when it comes to what "happened" (before eyes of human observers, according to the story), I actually do believe Hercules existed. Killed wife. Was sentenced to ten labours. The last two were simply his bragging. Some of the other ones would have involved bluffs as well, but there is a big difference between believing what an official tells you (and as cousin to king Eurystheus, Hercules was one!) and wholesale inventing what you were supposed to remember from your grandfathers' and greatgrandfathers' days. I don't believe in the explanations, they are told in an inaccurate theology. Zeus and Hera never had a marital dispute about getting one or the other to help in cleaning up monsters. Hercules didn't marry Hebe post mortem. Hercules wasn't son of Zeus. For that matter, I don't believe that Vishnu appeared as Matsya, or that Enlil decided the Flood and his brother Enki decided to arrange an Ark. But I do believe the main story.

I do not believe the people invented a story and decided to pretend to their children this was simply memory, and especially not that this happened with the same outline for a global Flood three or four times independently, or that it was transmitted to people not having made that kind of decision. That's too incredible for me.

Hans Georg Lundahl
Paris
St. John Baptist Maria Vianney
9.VIII.2023

Vigilia sancti Laurentii Martyris. Sancti Joannis Baptistae-Mariae Vianney, Presbyteri et Confessoris, caelestis omnium parochorum Patroni; cujus dies natalis pridie Nonas mensis hujus recensetur.

* https://en.wikipedia.org/wiki/Manu_(Hinduism)