Not all ancient cultures have a flood myth. When Japan became exposed to other cultures they learned those cultures had global flood myths. As Japan had no such story in it's past, the Japanese used this as proof that Japan was superior, the highest point on the Earth, and must be heaven.
What I find interesting is how there are 2 street signs in the world view of those who use other flood myths as proof of the flood in Genesis - One Way and No Turn. Why is it that other cultures global flood myths are valuable as proof of Noah's Ark, but the global flood in Genesis is not proof of the Epic of Gilgamesh?
In addition, those same cultures often have with their flood myths a creation myth. Yet, those creation myths receive no attention from those who look to bolster the story of Noah. It seems that only the Bible and Christianity can be true. All other religion or mythology is only good for providing evidence to support the exact same fantastic claims made by Christians.
There is a term for this phenomena - Confirmation Bias.