This, according to the Bible, was a world-wide deluge, but there is no record of any such phenomenon, of course. The Egyptian civilization, for instance, was building its pyramids. Nor do the Egyptian records speak of any floods other than the annual overflow of the Nile, as far as we know.
This is not to say, however, that the story of the Flood was not based on some actual, but local, flood in Sumerian history.
Summeria was a flat land between two large rivers. As is true of any large river unusual rises will bring about flooding conditions. In a country as flat as Sumeria, it would not take much of a flood to cover large portions of the entire region.
A particularly bad flood would live on in the memory of later generations, and particularly bad floods undoubtedly occurred. [...] the evidence exists that somewhere about 3000 B.C. there were indeed drastic floods of at least a local nature.
With time, as the storyis told and retold it is dramatically inevitable that a flood which spreads out over parts of Sumeria and neighboring regions with great loss of life will be said to have covered "all the world," meaning the entire region. It is further inevitable that later generations, with a much broader knowledge of geography, would accept the pharse "all the world" literally and reduce themselves to needless speculations on the impossible.
(A well-known example of this is the statement frequently met with among the ancient historians that Alexander the Great "conquered the world" and then wept for "other worlds to conquer". What was meant was merely that Alexander had conquered a large part of those sections of the world which were well known to the Greeks of the time. Actually, Alexander conquered only 4 or 5 per cent of the earth's land surface and had plenty of room in which to extend those conquests.)
(Asimov's guide to the bible, pages 38-39)