Origin of flood myths
With stories of some kind of cataclysmic great flood found in so many different cultures that were completely separate from one another, people throughout history and up to now have naturally speculated on whether they were all actually describing the same event. Most credible scholars today do believe that the mythological foundations for these legends originates from actual flooding events that occurred around the very tail-end of prehistory; long enough ago that there wouldn't be any written records or other methods of direct documentation to reference, but recent enough that the cultural memory and passage of the tale down through oral history could've formed the flood myths retold to this day. However, scholarship is divided with regards to whether it was one global-scale deluge event that inspired these tales or a series of more localized floods in the regions these cultures inhabited.There isn't a whole lot of evidence for the former, but one compelling candidate might be the sudden outflow of glacial Lake Agassiz
at the end of the last ice age; a large section of the glacier enclosing it broke off and dumped nearly all of its water into the ocean, which would've seen sudden floods along coasts worldwide along with strange extreme weather events caused by abrupt changes in the ocean currents that people at the time could not explain. In addition, the gradual but drastic rise in sea levels caused by the receding of glaciers during the end of the last glacial maximum in general submerged quite a few tracts of land that humans had inhabited beneath the sea, with Doggerland in the North Atlantic being one of the most striking examples, as bones, tools, and even primitive structures from the sunken landmass are still being dredged up today.note
As for the latter, every region of the world has seen some kind of mass-flood in the (geologically) recent past; in the Eastern Mediterranean, there are two of note, one which saw the Black Sea suddenly flooded around 5600 BCE, and the other being the volcanic eruption of Thera and resulting apocalyptic tsunamis felt across the Mediterranean in 1600 BCE. Similarly, with regards to African, South Asian, and indigenous Australian versions of the myth, studies of the Burckle Crater in the Indian Ocean have revealed that a large comet impacted sometime around 2900 BCE and would've also produced cataclysmic "megatsunamis" that would've struck every coast bordering the ocean. And in North America, the melting of the ice sheets that previously blanketed much of the continent during the last ice age produced terrifyingly huge outburst floods in some places like the Missoula floods in what's now the western US roughly around 10,000 BCE, which produced torrents of water as high as 120 meters and moved at speeds of up to 130 km/hnote .
A more mundane local origin could also simply lie with the rivers that the civilizations and societies of these early peoples basically depended on for their survival. From the Tigris-Euphrates river valley of Mesopotamia and the Near East to the Yellow River in China to the triple-river junction upon which the ancient South American Norte Chico civilization was built, all were constructed directly upon the floodplains of these rivers. It was the reasonable thing to do as an agricultural civilization, after all, who doesn't love free crop irrigation? But the tradeoff is that these rivers will naturally flood come significant rainfall or snowmelt, and the floodwaters will take everything along the floodplains with them. It may not sound as dramatic as a giant continent-smothering torrent of water, but to ancient people, they would've lost their whole world to the floods. Besides just their homes, their entire yearly crop yields could be swept away in a few moments, and when that was the thing you survived on back in the day, it would certainly feel apocalyptic.
