In the words of the apparently-not-such-a-good-guy-after-all Bill Cosby, "How long can you tread water?" (from his "Noah" routine if you're not a Bill Cosby fan...like I USED to be).
I think there might be a originating back story to folk lore that morphed over the centuries to become the myths and legends that are still around today, but if we want to know where they came from we might have to take a university course in the subject.