In these times, very few people would travel beyond the next villageand 99.9% of people probably never travelled further than 50 miles from where they were born unless they were nomadic.
So, If a major flood occurred (don't know how before climate change), it would seem to them that the whole world was flooded.
it's easy to prove that all the water on earth cannot flood the higher mountains.
However, the Holocene global warming caused a 135 m ocean rise within a few millennia, and so, many local floods here and there according to successive meltwater pulses.
Middle-East regions (where the Flood myth first appeared in Sumer) have been affected by 2 major events:
- the rapid formation of the Persian Gulf
- the catastrophic formation of the Black Sea.
Atheists have no proof of that, but geologists do have proofs of regional floods.
Each one could have inspired the Sumerian account of Gilgamesh Flood (the original of Genesis Flood).
Centuries of oral traditions exaggerated a regional catastrophe into a global gods'anger.
On the contrary, I have, or could easily find, “proof” that there have been lots of floods.
If you are referring to the so-called “great flood” and you've simply expressed yourself badly, I have no idea whether such a thing happened or not (I care even less) but there would be nothing at all unusual in a flood.
You don't need any “proof” to know that it didn't happen as described in The Bible though, all you need is a little common sense.