Different cultures and religions all over the world have similar stories: the Hopi, Christians, Inuit, Incans, the Chinese, Sumerians, Scandinavians, and Filipinos. This would seem to indicate that there actually was a more global flooding event. A major suspect for such an event would be the thaw following the last ice age which caused sea levels to rise very rapidly, inundating coastal lands where many humans had settled. This would have been prehistoric (~10-12,000 years ago) but may have been passed down through oral traditions, possibly explaining the origin of many of the flood legends and myths.

Just check out this map of Earth's land mass during the last Ice Age