The theory mentioned runs along the lines of:
All the earth’s water was at one time in vapor form. The earth was surrounded by thick fog clouds, which subsequently blocked most UAB rays, making cancer virtually nonexistant.
Since we had virtually no exposure to oxidizing radiation, man was very healthy as a species and lived very long lives. Probably grew much bigger as well, giving rise to the giant races, found in the bible and elsewhere in mythology.
Consequently, animals, too were healthier, and grew to monstrous proportions. Dinosaur bones are nothing more than modern day animals, but in these gigantic sizes.
The great flood came when the atmospheric temperature dropped and all the water condensed, forming the oceans and flooding most of the earth.
As a result, we lost our protective blanket against the sun’s rays, and our lifespan has diminished accordingly, due to harmful exposure to the sun’s radiation.
Not saying I believe it, but you asked to hear the theory.
There’s also the theory that leading up to the Deluge the earth was in a glacial period and very dry causing lots of problems. Then suddenly one day the ice sheet slipped off Antarctica due to heat and pressure under the ice and flooded the entire world.