And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
Genesis 1:2
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.
It’s the Biblical conception of Earth. That’s why.
what’s in the great deep?
Not much, apparently?
Here’s the two references to it in the Bible:
Genesis 1:2
Genesis 7:11
So… dead fish? I have no idea.
And some broken fountains, apparently