I think that it did occur (naturally since this story is represented in some form in many of the cultures in and around central Asia and the Middle East) but I think it was much more local than the Bible describes. Also, there isn't any evidence in the global geological record of a worldwide flood. There's a theory that this actually occured in the area around the Black Sea and was due to a glacial dam that broke and released a massive amount of water into the surrounding areas. This also explains the brackish water table under the salt water on the Black Sea.