According to Genesis 6:9-9:17, “Noah was a righteous man and walked with God. Seeing that the earth was corrupt and filled with violence, God instructed Noah to build an Ark in which he, his sons, and their wives, together with male and female of all living creatures, would be saved from the waters. Noah entered the Ark in his six hundredth year, and on the 17th day of the second month of that year the fountains of the Great Deep burst apart and the floodgates of heaven broke open and rain fell for forty days and forty nights until the highest mountains were covered to a depth of 15 cubits, and all earth-based life perished except Noah and those with him in the Ark.”
The Bible is not a history book, so perhaps we should take such a story as a myth rather than a record of an actual historical event. Interestingly though, the Bible is not the only place where you can find such a story of the Great Deluge. The Bible is not the oldest source of the story either. A pretty similar story can be found in the Epic of Gilgamesh, the Sumerian Epic poems about Gilgamesh, king of Uruk (an ancient city of Sumer), which predates the Torah (Jewish Bible). Similar Great Deluge stories also appear in many other cultures that include Indian, Chinese, Greek, Norse, Polynesian, and Mayan mythologies. It makes you wonder why. Could it be that such a global scale cataclysmic event might actually have happened?
I think I can answer that question. My short answer is highly unlikely. I can prove at least that it couldn’t have happened by raining as described in the Genesis using some scientific data and simple arithmetic. First, we need to find out how much water is in the atmosphere. According to an article by US Geological Survey, there is 12,900 cubic kilometers of water in the atmosphere. (The original source of the data is Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World’s Fresh Water Resources, Oxford University Press, New York.) Since 1 cubic kilometer = $2.642\times 10^{11}$ gallons, 12,900 cubic kilometers = $3.40782\times 10 ^{15}$ gallons, i.e. 3.40782 million billion gallons of water in the atmosphere! That is a lot of water but it counts only 0.001% of total Earth’s water. Let us assume that all that water in the atmosphere will turn into rain. Now, the question is how deep would the rainfall be if it were to cover the entire surface of the Earth like the Great Deluge allegedly did in Genesis? The mean radius of the Earth is 6,371 kilometer, so the total surface area of the Earth is $4\times\pi\times (6,371)^2\approx 510\times 10^6$ square kilometers. Dividing 12,900 cubic kilometers by 510 million square kilometers, we have $2.5294\times 10^{-5}$ kilometer = 0.995827 inch since 1 kilometer = 39370 inches. Of course, the Earth’s shape is not a box, so, rigorously speaking, the calculation is not correct. However, the depth is so small compared with the radius of the Earth, the above calculation won’t make much difference from the accurate one. In fact, the accurate depth $x$ is found by solving the cubic equation $\frac{4\pi}{3}[3(6371)^2x+3(6371)x^2+x^3]=12900$. It has one real solution $0.000025$ kilometer=$0.98425$ inch (and two complex solutions). The rainfall depth is mere 1 inch so it could never have caused the Great Deluge that would have covered even the highest mountain in the world.
I am not, by the way, claiming that the Great Deluge, or something to that effect could never have happened. All I demonstrated was it could not have happened by raining as described in the Bible. But such a cataclysmic event could possibly have happened by other means. For example, a huge comet or asteroid strike on the ocean would result in a super megatsumani possibly causing a great deluge worldwide. Such a comet or asteroid impact on the ocean will also increase water vapor in the atmosphere that results in an increase in rain significantly worsening the flood.
