Genesis 7:12

And the rain was upon the earth forty days and forty nights. The exact fixing of the day on which the terrible punishment of God had its beginning serves to emphasize its importance for all time. It was not a small local disturbance which is here recorded, but a universal deluge, a flood which covered the entire earth. It was a miracle of God's avenging and punitive justice. For all the fountains of the great deep were split open and broke forth: the waters beneath the earth which are otherwise locked and hidden in its depths rushed forth with impetuous force. And the windows of heaven were opened. The waters which are ordinarily held back by the firmament set above by the Lord, Genesis 1:6, were now released to pour their masses upon the earth. At the same time a rain set in which streamed down without intermission for forty days and forty nights.