Christians who take every part of the bible literally, the flood according to the bible happened a little over 4000 years ago. A global flood like that 20 feet over the tallest mountain. Your water 4 miles deep. A flood like that would have ripped the earth to pieces and flattened every thing. The oldest man made structure go back 10's of thousands of years none of them would have survived the flood. Geology, biology, physics, and astronomy and carbon dating (Advancing technology has allowed radiocarbon dating to become accurate to within just a few decades in many cases). The bible is the history and myths of the Jewish people. That does not mean god is not real but it does mean, the earth is not flat, diseases were caused by germs not evil spirits, the earth is not the center of the universe. The people of the ancient world believed what they believed out of ignorance. Now as for a massive destructive flood like the Black sea flood, that is probably the truth of the flood that every culture on the planet has a story for. But as for the bibles version of the flood it did not happen.