Let me begin by showing what the Bible says about the flood:
Gen 7:10: And it came to pass after seven days, that the waters of the flood were upon the earth. 11: In the six hundredth year of Noah's life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. 12: And the rain was upon the earth forty days and forty nights. 13: In the selfsame day entered Noah, and Shem, and Ham, and Japheth, the sons of Noah, and Noah's wife, and the three wives of his sons with them, into the ark; 14: They, and every beast after his kind, and all the cattle after their kind, and every creeping thing that creepeth upon the earth after his kind, and every fowl after his kind, every bird of every sort.
I believe that there was a canopy suspended above the earth that caused a type of greenhouse effect. This explains the life spans being much longer because you were protected from the elements, and lived in a perfect climate. The plants of the earth were watered from the springs below which would flow up from the ground (12).
Read the following from Gen 1:
6: But there went up a mist from the earth, and watered the whole face of the ground.
To flood the earth God simply dropped the canopy from above and the waters from below exploded from the ground. This was not a regular rain storm, it was a catastrophic event. (12)
Here are some points to consider:
When a fish dies its body floats on the surface or sinks to the bottom and is devoured quickly, usually within hours by other fish. Why are fossilized fish found in sedimentary rock preserved with all bones intact? Fish found over large areas numbering in the billions have been found in a state of agony with no mark of a scavengers attack. (7)
Estimates run as high as 5 million mammoths remains are buried all along the coast line of northern Siberia into Alaska. A freak accident is the only explanation that science can give. (7)
Only a flood could result in finds that show a mixing of plants, animals, and insects from entirely different habitats and regions. (7)
In summary the preservation of organic materials such as fossils requires some sort of catastrophic condition, some kind of quick burial by engulfing sediments, usually followed by some means of rapid hardening. (7)
Lets look at how many different lines of evidence of a historic nature point back to the Biblical time of the flood being 4-5000 years ago:
The oldest trees found date back to a time 4000 years ago(bristlecone pines), scientists agree that these trees seem immune to everything except catastrophe. Why have none been discovered that date back before the flood? There has never been a living thing discovered that dates back before the Biblical date of the flood.
Sequoia gigantea trees may enjoy perpetual life since they appear to be immune to pest attacks. But still none have been dated back before the flood.
I covered this in the age of the earth section but I will mention it here also: The population of the earth doubles about every 175 years. Simple math will allow you to take the population of the earth today and cut in half every 175 years as you work back. You will reach the end at about 3300 BC, the birth of Noahs first son.
(ref: The Genesis Flood - John C Whitcomb/ Henry M Morris)