I ran into someone who tried to run through some calculations to prove that a world-wide flood could not have happened due to there not being enough water for condensation to cause enough rain to fall. I had to explain to him where the water came from, and it was quite a surprise to him.
The Bible talks about the process of condensation in Job 37:27-28. It describes how dew is drawn away from the ground into the air and into the clouds and later turns into rain. And this was written thousands of years before scientists understood it. It was not until the 1600s that science was able to explain how the process worked. But God had it recorded in his word long before man figured it out.
The earth's environment before the flood is a mystery to most people. But Scripture tells us that there seemed to be an ocean in the sky before Genesis 6. We're told....
6 And God said: 'Let there be a firmament in the midst of the waters, and let it divide the waters from the waters.' 7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. 8 And God called the firmament Heaven. And there was evening and there was morning, a second day.
So while we look up and see clouds in the sky, the ancients looked up and saw an ocean in the sky held there by some sort of firm structure God had suspended. In fact the name "Heavens" or "שמים" seems to come from this and possible means "שם" / "there" plus "מים" / "water" or "water there", as if to say that when they looked up, they called that thing in the sky "water there" when they called it "שמים" or what we call "the heavens". The ocean in the sky may be gone, but the name giving rise to calling it "שמים" (water there) has stuck.
This water in the sky would have made the environment a lot healthier. As can be seen from the photo on the left, cosmic radiation would have been shielded instead of passing through. The atmosphere would have had a much higher oxygen content, and this is why people before the flood lived to be 900 years or more, whereas after the flood people slowly began dieing at significantly younger ages.
Perhaps the long life was part of what made men violent back then and led to God deciding to shorten lives to avoid such a thing. Also, I have heard of some archeological evidence in Christian circles that men did not stop growing back then. They didn't grow to 6 foot and then stop, but continued to grow as long as they lived, so someone 900 eyars old would be significantly taller than someone 50 years old. That gave really old people an advantage in a violent conflict. It that theory is true, it explains why violence would lead to eliminating the water canopy, because it eliminated the advantage someone would have just because he is old and levelled the playing field much more.
Water from condensation was not the only source that would have caused the flood. The water canopy above was punctured to let the water drip out all over the earth over a 40 day period. Flooding also came up from the ocean floor, since we're told....
"In the six hundredth year of Noah's life, in the second month, on the seventeenth day of the month, on the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. And the rain was upon the earth forty days and forty nights. " (Gen 7:11-12)
So flooding came up from the ocean as well as down from the sky. How much water was involved is hard to say, we can't calculate it today because all we know is our present environment and things were much different back then. Once I showed the guy this, he had no choice but the realize that his presumptions were wrong and perhaps the Bible was telling the truth.
Some people claim a flood happened , but it wasn't global, based on some of the same reasoning. Again, this fails to account for the fact that no condensation was really needed to produce the flood waters.
The above photo comes from http://www.hydrogen2oxygen.net/en/tag/water-vapour-canopy/ where it explains why Carbon dating prior to times before the flood will produce results WAY OFF reality, because Carbon dating does not take into account what the environment was like before the flood. In fact, the fluctuations of C14 to C12 make it less than perfectly reliable in today's standards for that matter for a variety of reasons. Keep an artifact in a glass case or an airtight vault or something and it may not age as fast as something out in the open. Carbon dating has so many presumptions going into it that you wonder why scientists even bother to use it. I can remember when I was in Engineering school, most of the class laughed at our professor once when he raised the issue and talked about how unreliable it was. He tried to form an apologetic excuse for the flaws in the methodology, but belief in it is by no means a unversally excepted concept among all scientists. Don't let liberal press tell you otherwise.