And God said, “Let there be an expanse between the waters to separate water from water.” So God made the expanse and separated the water under the expanse from the water above it. And it was so. God called the expanse “sky.” (Genesis 1:6-8)
When we take these statements literally—as it has often proven difficult for literally-minded interpreters to do—they have profound cosmological implications. According to the Genesis account, God’s creative activity on the second day consisted of inserting a hollow, but solid, dome-shaped object in the midst of the waters, with the result that some of the waters were then “below” this dome while the rest were “above” it.
The Hebrew word for this object is raqiya, derived from a verb meaning “to spread out” or “to beat thin” (as one might do with metal). The picture is of a tent being stretched out on poles, or of metal being poured or pounded into the shape of a dome. This was the Hebrew understanding of how God had made the sky. Thus Elihu challenged Job, “Can you join God in spreading out the skies, hard as a mirror of cast bronze?” (Job 37:18). Several other Old Testament writers similarly refer to God “stretching out” the sky (Psalm 104:2, “like a tent”; Isaiah 40:22, “like a canopy”; Jer. 10:12, Zech. 12:1). Further evidence that ancient peoples understood the sky to be solid may be found in the account of the Tower of Babel, whose builders expressed the expectation that they could build right up to the dome (Gen. 11:4).
The King James Version renders this word as “firmament,” but a solid object is definitely in view. (The latest update to the NIV  changes its previous reading “expanse,” quoted above as in the printed version of our book, to “vault.”)
A solid sky is already one significant departure from our modern cosmology. But there is another: The author of Genesis clearly believed that there was a quantity of liquid water above this solid sky. From the text, there is no reason to conclude that these “waters above” were in any way different from the “waters below” (the seas). In other words, the Genesis author is not describing water vapor or clouds, as a modern reader might surmise. Those would be waters in the sky (or more literally before the face of the sky, i.e. just this side of the dome, which is where the birds are said to fly in Gen. 1:20). This account instead describes waters above the sky, which we should understand as the “seas above,” that is, the remnants of the primordial watery chaos.
As the book of Genesis continues, we discover that some time after the creation, these “waters above” are called upon to serve God’s further purposes. Wickedness has spread pervasively among all of the creatures on the land, and God has determined that He must destroy the creatures by destroying the land on which they live, in order to restrain this wickedness (Gen. 6:13). To generate the flood that swamps the dry land, God opens the “windows of the sky” (Gen. 7:11, 8:2) and lets the “seas above” flow through the dome on to the dry land.
The term translated “windows” here refers to an opening such as a ceiling lattice through which smoke escapes (Hosea 13:3), or the holes in the side of a dove cote that let in light and air (Isaiah 60:8). It is not used in the Hebrew Bible to mean “window” in the sense that we usually understand that word, an opening in the wall of a human habitation. (Indeed, a different term is used for the “window” of the ark in Gen. 8:6.)
We may rather understand it by analogy to the floor grates built into old houses that were heated by basement coal furnaces. The furnace would heat the air of the entire basement and the floor grates were then opened to let this warm air rise throughout the house. But perhaps the best analogy we can draw is to the floodgates built into dams that allow water backed up behind the dam to be sent downstream, either for irrigation or to keep the dam from overflowing. The “windows of the sky” in Genesis 7:11 and 8:12 are literally “floodgates” that allow the “seas above” to flow through when opened.
The vision of the second day of creation, therefore, is of a hollow but solid dome with seas beneath and above. To think of the “firmament” or “expanse” of the sky as a gaseous atmosphere is to read our present understanding anachronistically back into an ancient text.