Early civilizations were generally built on great floodplains (the Nile, Euphrates, Indus), and floods were a regular part of their cultures. The annual (Nile Flood)[http://en.wikipedia.org/wiki/Flooding_of_the_Nile] has heights of 20-50 feet on average. A 100- or 1000-year flood would be an epic event.

Ditto for the Indus (which most recently had a disastrous flood in 2010), Tigris-Euphrates, Ganges (which has disastrous floods so regularly that it barely registers as news), and other great river systems. We can say with certainty that great floods occurred in these cradles of civilization, because even their regular annual floods are major events, and statistically some of these will rise to the level of epic natural disasters, just as they continue to do today.

I have never seen evidence of a coincidence of great floods across all of these river systems at once. That would not be necessary to establish a myth, however. It's not like someone in the Ganges in 5000 BCE would be aware of what was going on in the Nile at the same time. No matter which river system you lived on, a 100- or 1000- year flood would submerge your entire known world.

Great comments here on floodplains, etc. I find the idea of a Black Sea basin sudden flood of interest. That said, we do not need an event (or events) to explain stories in ancient or modern oral traditions. And similarities between international bodies of oral traditions do not imply that there was an actual core event. Similarities in the human existence and in the nature of the world lead to parallels in oral traditions.

Noah-flood enthusiasts look at the widespread nature of these stories and use a flawed form of logic to conclude that the Biblical version is therefore exactly how this mythical flood occurred. The first flaw is in concluding that widespread stories are based on fundamental truth. That would mean that all the stories about culture heros - stories that have many shared elements - are based on a single hero that is remembered by all cultures. There is simply no reason to reach that conclusion; besides the fact that it would be an absurd construct, it is clear that people tell stories that are cultural inventions, and that many of these stories seem to have counterparts internationally. This line of thought would conclude, based on international evidence, that there was a proto-Cinderella, a young woman who was treated poorly by a stepmother and her family, but who rose to marry a wealthy young man. I really don't think we would want to go there.

The second flaw in the logic here is the idea that the Biblical version of the flood story is the most accurate. It is not the oldest (why not look to the Gilgamesh version as the most accurate since it is far older?). The idea that the Noah story is the best version is the height of ethnocentrism. I understand that it is based on the belief that the Bible is the literal word of God. That's fine for those who believe that, but the deduction that Noah was the actual survivor of the flood, then, mixes a pseudo scientific methodology (find international variants of a story and conclude that there was an international proto-event) and then see this as verification of the validity of the Noah story (and "our" version is clearly the best because I have faith that it is the literal word of God). This mixture of an attempt at scientific deduction requires a leap of faith, at the end, to conclude that Noah was our man.

Scientifically, if we accept the first dubious point (international counterparts prove flood), then we must, logically, look to the oldest version or at least to the most shared motifs internationally, to reconstruct the circumstance of this imaginary proto flood.

edited for typos.

Earth came out of an ice age period around 12,000 years ago. The glacial melting raised the sea level a significant amount and nearly all coastal regions would have been wiped out. It is possible then that these myths are remnants of events that could have slowly occurred while this glacial melting period took place.