[Tulac]

The Great flood probably hapenned, because it is described in many books from the old ages, but it must've been some great flood, not that caused by God but something naturaly, it was such a great cataclysm, that people all over the world knew about it...
And many of the Holy books explain pretty much the same events in a different manner, that is a well known fact...
Those are legends that have been re-told, so many times that they got distorted, and everybody saw it in their own perspective, by what their ancestors told them...
Some natural catastrophies, and other 'odd' events for that time were hapenning thousands of years ago, before christianity, and not only christians speak about it, but muslims, and other religions too(even Dogons who are a tribe in Africa, and lived isolated since 200 years ago)...
So these Holy books were used to write down unexplainable legends from the past, to make them more interesting to the read, so they can incorporate their rules, with the help of these legends...