Actually, the Fire that IGNITES the fire to burn up "hell" (the graves) is God's all consuming Fire of His Eternal self. Therefore, once all the earth, the unsaved, and the works therein (grave digging is a work) are burned up by fervent heat, the Lake of Fire (molten lava over the entire globe) will leave nothing left.
KJV 2 Peter 3[6] Whereby the world that then was, being overflowed with water, perished:
[7] But the heavens and the earth, which are now, by the same word are kept in store, reserved unto fire against the day of judgment and perdition of ungodly men.
[8] But, beloved [on another topic], be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day [the current Age of God's Grace].
[9] The Lord is not slack concerning his promise [of His salvation], as some men count slackness; but is longsuffering to us-ward, not willing that any should perish, but that all should come to repentance.
[10] But [back to the first topic] the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up
When there is nothing left (no fuel for the fire) that fire, ignited by His Eternal Fire of Himself, will go out.