Did you read my post about an epic battle between the Native American Gods, Skell and Llao which is believed to be a dramatization of an event in history 6000 years ago. Or the "mythical beasts" that have been either found, or demonstrated to have recently gone extinct. Even the Duck Billed Platypus was initially approached with some skepticism.
Perhaps not myths per-se, but there are some very strong scientific reasons why incestuous relationships are a very bad idea.
Likewise, it has been demonstrated that other taboos such as cannibalism is unhealthy.
There is still some debate about unicorns, but it is quite likely that unicorn horns were modelled after either antelope horns or more likely narwhals, the unicorns of the sea.
Noah's Flood? Again hotly debated. There have been many floods. But, perhaps there is some cultural memory of sea level rises the flooding of the Mediterranean Sea in the early Holocene.
well to the Noah's flood during the biblical times there were believed to be heavy plate movement so rapid or advanced shifting of the earths landmasses far faster then they move today could cause untold or never heard of before or "biblical" like disasters.
A great example would be a hypercane no they don't exist in today's world but they could if the right conditions were possible http://en.wikipedia.org/wiki/Hypercane while this in itself isnt really a myth some people speculate that if Atlantis was real this could be the horrible storm that sunk or destroyed it.