"In the six hundredth year of Noah's life, in the second month, on the seventeenth day of the month, on that day all the fountains of the great deep burst forth, and the windows of the heavens were opened. And rain fell upon the earth forty days and forty nights."
Genesis 7:11-12
The first day of the Flood.
So what replaced this water during said flood?
That “Great Flood” was probably the result of an impact from an object from outer space, or a great Tsunami from an Earthquake. Those would be the most likely explanation for such a flood, assuming that it happened. I believe there is physical evidence that it did.