I've been reading the book of the Hopi recently. They have several flood stories in their mythology in which their people are wiped out for losing faith in god and letting "the holes in the tops of their heads" close up, through which god supplied them with knowledge. Many other cultures have flood myths as well, but I haven't gotten around to reading them yet.
I remember as a kid, watching some documentary on Noah's ark and scientists talking about how their is geophysical proof that at some point there was a flood. My father saw this as just one more reason that his bible was right. Of course, it also proves that the Hopi and the Chinese and the Babylonians were right as well, but I doubt he saw it that way.
When I read the stories of other non-christian cultures I find them surprisingly, well, Christian. The names of the gods are different, but the morals are still the same. I stopped reading the Tibetan Book of the Dead because I found it annoyingly similar to the bible (well, in some ways).
I find these myths very interesting when you look at them as proof that something happened, and all these different cultures explained it in their own way, adding embellishments as they saw fit. Behind these myths are very strong messages about morality, or what was considered "morality" at the time, which usually had to do with survival (ie: don't be homosexual because we need our clan to reproduce, don't lie with strange women because you'll get a disease)
And, if you've ever read Lord of the Rings or Dune, you know how comparitively easy (that is, if you're a literary genius like Tolkien or Hebert) it is to create an entire world and mythology with very few holes in it. One person could easily accomplish this in his lifetime. If you have more than one writer, well, just tack Revelations onto the end of your story to give the mathematician with the penchant for eating funny mushrooms something to do.