The 2nd heaven isn’t directly mentioned by number in any book of the modern Protestant canon. But it’s existence is evident when a 3rd heaven is mentioned in 2 Corinthians 12.
Contrary to popularly held teachings, this 2nd heaven is not a spiritual embodiment of an “outer space” in a heliocentric universe. Rather, the foundational definition of a “heaven” remains consistent for every heaven God created: a vaulted, firmament dome structure.
(See Genesis 1; On day two of creation, God named the firmament “heaven” in verse 8 – after already creating multiple other heavens on day one in verse 1.)
In the Testament of the 12 Patriarchs, Levi describes a vision of the 2nd heaven where he specifically sees fire and ice reserved for the day of judgement. These heavenly weapons are mentioned numerous times in scripture.
Testament of Levi 2:8-9
And further I saw a second heaven far brighter and more brilliant, for there was a boundless light also therein. And I said to the angel, Why is this so? And the angel said to me, “Marvel not at this, for thou shalt see another heaven more brilliant and incomparable.”
Psalms 18:12-14
From the brightness before Him, His thick clouds passed with hailstones and coals of fire. The Lord thundered from heaven, And the Most High uttered His voice, Hailstones and coals of fire. He sent out His arrows and scattered the foe, Lightnings in abundance, and He vanquished them.