This was accomplished precisely according to 2 Chronicles 36:19 and Jeremiah 52:13. The fires of judgment burnt down Jerusalem completely. But as soon as the destruction of the city was over, the fires went out. They extinguished themselves back in the 6th century B.C.E. They were "unquenchable" back at that time in the sense that no one was able to put them out until they had performed their prophesied duty. Likewise, Isaiah said that in a future time (Isaiah 66:24), the carcasses [dead bodies] of people who rebel against God will be given over to maggots for consumption. These worms will not die until they have consumed the bodies assigned to them. Other dead bodies will be set alight and "neither shall their fire be quenched" (Isaiah 66:24). These fires are prophesied to destroy the bodies of certain wicked people and God will not allow anyone to put out the fires — they are unquenchable. Obviously though, the worms are not immortal and the fires will finally go out of themselves when they have accomplished their tasks.
Needless to say, the fires in the Valley of Hinnom (Gehenna) were not eternal — they have long ago ceased. I have taken over 600 people on tours to Jerusalem and all of us have walked through Gehenna or driven cars through it. There are no longer any fires there. Indeed, there is a prophecy in Jeremiah that "the whole valley of the dead bodies [Gehenna]" will be made "holy unto the Lord" (Jeremiah 31:40). The prophecy shows that the burning of dead things in the region will permanently stop. Gehenna will even become "holy." This demonstrates that even the symbol of Gehenna as a place where dead bodies and refuse are burnt will cease as a fit type of God’s destructive power.
The third and final New Testament word translated "hell" is Tartaroo. It is found only in 2 Peter 2:4. The Greeks understood it to be a subterranean area designed as a prison for the Titans (giants, see Genesis 6:4) who rebelled against Zeus. These spirits possessed men before the Flood and caused the sons of God ("righteous men") to marry the daughters of men ("wicked women") and giants were born to the women. These giants would answer to the Titans of Greek mythology. All the humans died in the Flood (except Noah’s family), but the spirits who caused the mischief were consigned to the subterranean areas of the earth. Not being humans they could live in such a place. This may be the region referred to by Peter when he said that in the days of Noah Christ "went and preached unto the spirits in prison" (1 Peter 3:19). That such a prison for spirit beings does exist under the surface of the earth is attested in the Book of Revelation.
"And I saw an angel come down from heaven, having the key of the bottomless pit [Greek: abyss — the underworld or lower regions of the earth or oceans] and a great chain in his hand. And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him [as in a prison] a thousand years."
• Rev. 20:1–2
That there are beings under the earth is mentioned in Philippians 1:10, "That at the name of Jesus every knee shall bow, of things in heaven, and things in earth, and things UNDER the earth." In Exodus 20:4 (in the Ten Commandments) this subterranean area is associated with the oceans. It speaks about fish and other aquatic animals being there. But the area is also called an abyss or pit. Could live beings live there? It may seem strange to us but Revelation 5:3 has a reference to conscious beings in a place "under the earth". In Revelation 9:2 we read that spirit beings in this region will be able to come to the surface of the earth. But at the present time the area of Tartaroo is considered a prison (1 Peter 3:19) where spirit beings are, or can be, kept in "chains" (Revelation 20:1–3). There is no information in Scripture that would indicate that these beings "under the earth" are suffering the pangs of fire. Such torment is reserved for the lake of fire which occurs later (Revelation 20:10).
Christ was able to preach to those in the "underworld prison" (Tartaroo) and His message is destined to take root even with these estranged beings, because Paul went on to say, "Every tongue [including those in "the underworld"] should confess that Jesus Christ is Lord, to the glory of God" (Philippians 2:11). God will, through Christ, reconcile all things to Himself (Colossians 1:20). Even those in Tartaroo (the underground prison) will share in the reconciliation when they confess that Christ is truly Lord and rejoice in the fact that He is the Savior.