‘But the heavens that now are, and the earth, by the same word have been stored up for fire, being reserved against the day of judgment and destruction of ungodly men.'
And he wants all to realise that in the same way as that Flood arrived and took the world that then was by surprise, so in the future will God's fiery judgment come and take men by surprise, once His restraining word is again removed. It is His word that is keeping back the world's fiery end (He is ‘upholding all things by His word of power' - Hebrews 1:3). And it is that same word which will bring it on both the heavens and the earth when it finally comes. For the earth is stored up for fire, being reserved by God for that very purpose, that is, for the day of judgment and destruction of ungodly men.
It is probable that Peter has in mind here God's judgment on Sodom and Gomorrah as a forerunner of what is to come. In 2 Peter 2:5-6 he had spoken first of the Flood and then of the destruction of Sodom and Gomorrah by fire. And now here we have the destruction by the Flood, followed by destruction by fire. So he clearly sees that prior destruction by fire of Sodom and Gomorrah as being a kind of foretaste of what is to happen at the end of the world when all men of lust will also be consumed by fire, as they were then in microcosm (compare Ezekiel 38:22, ‘I will rain upon him (Gog), and on his hordes, and on the many peoples who are with him, an overflowing shower, and great hailstones, fire and brimstone').
He had good reason to do so. The Scriptures regularly depict the coming end in those terms. Joel, for example, foresaw a time when God would visit the world with blood, and fire, and pillars of smoke (Joel 2:30; compare 2 Peter 1:19-20; 2 Peter 2:3), words which were brought to mind by Peter himself at Pentecost (Acts 2:17). The Psalmist depicts God as coming with a devouring fire preceding Him at the time when He comes to judge (Psalms 50:3). Isaiah speaks of ‘the spirit of judgment and the spirit of burning' when God purifies the world at the last day (Isaiah 4:4), and of a flame of devouring fire when He brings about His judgments (Isaiah 29:6). Indeed he declares that the LORD is to come with fire, to render His rebuke by flames of fire, so that by fire and by his sword He will plead with all flesh (Isaiah 66:15-16). Ezekiel declares, speaking of the last days, ‘I will send a fire on Magog, and on those who dwell securely in the coastlands, and they will know that I am YHWH' (Ezekiel 39:6). And Malachi says that the Day of the LORD will burn as an oven (Malachi 4:1). Compare also Amos 1:4 to Amos 2:2. Furthermore Paul had revealed in his letters the fiery nature of the judgment that was coming (2 Thessalonians 1:8). So with Sodom and Gomorrah already in mind Peter had good Scriptural grounds for considering that the final destruction must be by fire, as it had been for Sodom and Gomorrah. He did not need to look elsewhere for his ideas, although it was in fact supported by both Jewish tradition and Greek tradition.