In the six hundredth year of Noah’s life, in the second new moon, the seventeenth day of the new moon, on that day all the springs of the great deep were broken open, and the windows of the heavens were opened, Genesis 7:11.
When God renovated the earth for humans, He cleared the judgment that came upon Eden due to Lucifer’s sin. Preparing the earth for mankind, He caused a divide between the waters that covered the earth, placing them above and below the sky (Genesis 1:7). This created an environment similar to a greenhouse. At that time, no rain fell upon the earth. Instead, a mist watered the ground each morning (Genesis 2:6). Humans mostly stayed together near the Garden of Eden (Genesis 4:16).
The flood was caused by both the springs of the great deep being ripped open and the water above the sky falling down. At this time, the earth was one continent (Genesis 1:9) and fairly flat by today’s standards. Over the next forty days, the water rose to about 80 feet, sufficiently high enough to cover every mountain and drown every living creature on dry land—about 25 feet above the mountains (Genesis 7:20). With the natural rotation of the earth, the water above came down intensely fast at the poles, instantly freezing any animals caught in its wake, while those near the center of the earth would have been crushed by the water coming from the fountains of the deep.
Where does the idea of 80 feet of water come from? After the flood, humans rejected God’s promise and built a tower for themselves with reference to the heavens (Genesis 11:4). This tower was made out of brick and asphalt mortar, giving it a maximum height of eight stories (Genesis 11:3). The tower’s height, built in anticipation of another flood despite God’s promise, suggests their belief that a flood could reach such a depth (Romans 1:32). Since the earth was still one continent at this time, it is reasonable to infer that the flood waters could have reached a depth of about 80 feet, as the tower builders aimed to create a structure above any anticipated floodwater level.