Read Genesis 7
Focus on verses 11-12
Water bursting out of the ground… water pouring down from the heavens! – There is a theory that lends some credence to the idea of a young earth (as opposed to billions of years) called “the canopy theory” that suggest that these forty days and nights of rain is the first time the inhabitants of the earth experienced rain as we know it.
In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on that day all the fountains of the great deep burst forth, and the windows of the heavens were opened. And rain fell upon the earth forty days and forty nights.(v. 11-12)
This theory proposes that until this great flood, a layer of water like a huge canopy encircled the entire globe, and that prior to the flood the entire earth was not unlike a tropical greenhouse from pole to pole. It also suggests that the reason a man could live 600-900 years prior to the flood was at least in part because the surface of the earth was shielded by the water canopy from the harmful ultra-violet rays of the sun which cause aging to occur at a faster rate.
If this theory is true the flood occurred when the canopy collapsed… why and how remains a mystery beyond that God used it to bring about judgment upon those who lived on the earth. This deluge of water would have changed the geography and climate of the entire earth quite drastically. Perhaps this would also explain why the first rainbow is seen after the flood. It might explain why “rain” is not mentioned prior to the flood. It may also explain why archeologists reportedly have found tropical vegetation under the polar icecaps.
While I find this theory interesting, it is not necessarily one I would stake my faith upon. What is important to matters of faith in regard to the flood is that God was the One who orchestrated it. It was God who called Noah to build the ark, and who shut him and His family into it, and who kept his hand of protection upon them throughout the year long ordeal to bring them safely through it. The story is about God, and reveals to us a little more of who He is and how He relates to His creation. Anything that we might ascertain in addition to that about creation theory is just a bonus.