Mark 9:48

Where their worm dieth not, and the fire is not quenched.

Cross-reference

Matthew 18:8 similarly warns against causing sin by cutting off a limb to avoid eternal fire, paralleling the unquenchable fire of Gehenna.

Matthew 25:46 contrasts eternal punishment with eternal life, reinforcing the fate of the wicked in the unquenchable fire here.

Leviticus 6:12 commands a never-quenched fire on the altar — a pattern that points forward to the unquenchable fire of judgment.