Two things.
1.
Scientists Discover Massive Hidden Ocean 400 Miles Beneath Earth’s Surface
Scientist are only just beginning to learn about the hidden oceans under the earths crust. In the book of Enoch, for example, it says that God released the water from the deep.
2. The mystery of the firmament, that is, the preflood atmosphere.
It is a theological mystery as to what those waters above were, but are no more. Whatever it was, those waters more than likely came down and helped flood the globe.
In fact, ancient cultures around the globe have myth stories about the firmament, just like they do about giants and a great flood worldwide.