The earth was once surrounded with water, hence,
They deliberately forget that God made the heavens by the word of his command, and he brought the earth out from the water and surrounded it with water. (2 Peter 3:5 [NLT])
And so, this was the same water formed above the firmament in the beginning,
And God said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. (Genesis 1:6 [KJV])
In which case, a canopy of air was formed when the firmament came into existence with the waters above,
So God made a canopy that separated the water beneath the canopy from the water above it. And that is what happened: God called the canopy "sky." The twilight and the dawn were the second day. (Genesis 1:7 [ISV])
And this canopy came as a result of a layer of ice that surrounded the earth, which was the waters above the firmament, hence this vision of Ezekiel,
And the likeness above the head of the living creatures was as a firmament, as the vision of ice being stretched out over their wings on top. (Ezekiel 1:22 [ABP])
And it was by the melting of this layer of ice that the entire world became flooded,
By these waters also the world of that time was deluged and destroyed. (2 Peter 3:6 [NIV])
And rain fell on the earth forty days and forty nights. (Genesis 7:12 [NIV])
This layer of ice also created natural hyperbaric conditions in the sky by pressurizing the air and causing denser air richer in oxygen, hence why when this ice layer collapsed, the fountains of the deep gushed forth,
In the six hundredth year of Noah's life, on the seventeenth day of the second month--on that day all the springs of the great deep burst forth, and the floodgates of the heavens were opened. (Genesis 7:11 [NIV])
Because just like a soda bottle releases pressure when a bottle is opened, so was pressure released from beneath when the ice layer collapsed, thus causing the waters from beneath to gush forth upwards.
