The flood seems to be backed up by a few things:
1-Almost every culture has a flood story in written or picture form.
2- Fossil locations -- This one is very interesting to me since when I was younger we went to a place in Nevada called Fossil Mountain that is well over 5000 feet above sea level and there were a ton of fish fossils on the top of the mountain. It is a small crater where it would appear that water levels rose above it and then due to the concentration of fossils there were many fish and other animals trapped in the crater when the water level dropped back down.