I mean, yes, there was undoubtedly a big flood a long time ago (almost every culture in the world has stories about a Big Flood),
Actually, I've been thinking about this a lot lately, and at the risk of starting a BIG FREAKING TANGENT....
I don't think the presence of a flood story in a lot of different cultures' mythologies is actually proof that the world ever experienced a big fuckall flood in human history. I think it's actually just symptomatic of the way human societies form.
When you're a fledgling human society, just coming in from your hunting and gathering and trying your hand at agriculture, you tend to look for a few key things in a permanent residence:
-Appropriate climate for the type of crops you know how to grow
-Proximity to a RIVER
Now, funny thing about rivers: they flood. For most rivers, this happens extremely erratically. You'll just be minding your own business and then it starts raining and then *bam* you're up to your neck in water and your livestock is floating away.
And the rivers most prone to extreme floods are also the ones surrounded by the most fertile lands, as the flood drops rich silt in its course. So those same rivers were the most likely to attract successful societies.
Given that, is it really that strange that most human cultures that have successfully passed through their "agriculture-based" stage have this big "flood-to-end-all-floods" horror story lurking in their collective unconscious?
Now, let's consider an exception to this rule: Egypt.
Ancient Egypt, like most budding agricultural societies, sought out their fertile land, their appropriate climate, and their river so that they could start farming. But the Egyptians chose a rather interesting river.
The Nile, unlike many rivers, had a very *regular* flood cycle. It came like clockwork, every year. (I use the past tense because damming projects have somewhat changed things in the modern era) Egyptian farmers just incorporated it into their routine. Around the beginning of flood season, they stop tilling their fields (in the flood path), and just stay in their house (conveniently uphill from the flood path). When the floodwaters recede, the farmers return to their fields (which are now coated in a rich, luxuriant layer of silt) and begin planting the next year's crop.
For the Egyptians, the flooding of the Nile was a *blessing*. It was a signal that the gods *loved* them and wanted them to be rich and happy.
So do the Egyptians have a flood story?
But they *do* have a disaster story, and it follows the same sort of formula: God (in this case, Ra) gets angry; God sends disaster; Most of population dies; God decides not to kill *everyone*; Humans, suitably chastened by the display of power, repopulate and remember not to be so impolite to the gods next time.
Except with the Egyptians, the disaster story took the form of a huge Desert Lion (Actually the transformed goddess Hathor) that ate nearly all the humans on the earth before she was subdued and dragged back to A'aru.
So it looks, to my admittedly untrained eye, that the whole "Flood Story" trend is actually an attempt by early cultures to fill some "Vengeance of the Gods Story" niche, and just using one of their biggest and most ever-present fears to fill in the gaps. It just happens that most early societies happened to have the same collective fear.