Genesis 1:7 (BSB)

So God made the expanse and separated the waters beneath it from the waters above. And it was so.

From Genesis 1. Also in the ESV.

Commentary on Genesis 1:7

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Genesis 1:6-8: We have here an account of the second day's work, the creation of the firmament, in which observe, 1. The command of God concerning it: Let there be a firmament, an expansion, so the Hebrew word signifies, like a sheet spread, or a curtain drawn out.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Genesis 1:7: And God made the firmament,.... By a word speaking, commanding it into being, producing it out of the chaos, and spreading it in that vast space between the heaven of heavens and our earth (z), And divided the waters which were under the firmament from the waters which were above the firmament; the lower part of it, the atmosphere above, which are the clouds full...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Genesis 1:6-8: SECOND DAY. (Gen 1:6-8) firmament--an expanse--a beating out as a plate of metal: a name given to the atmosphere from its appearing to an observer to be the vault of heaven, supporting the weight of the watery clouds. By the creation of an atmosphere, the lighter parts of the waters which overspread the earth's surface were drawn up and suspended in the visible heavens, while...
• Geneva Bible Notes (Reformed), Geneva Bible Study Notes on Genesis 1:7: And God made the firmament, and divided the waters which [were] under the firmament from the waters which [were] above the firmament: and it was so. (f) As the sea and rivers, from those waters that are in the clouds, which are upheld by God's power, least they should overwhelm the world.