Sacred Flame

The sacred flame, located on the planet Karn, was the heat source that created the life-prolonging Elixir of Life, guarded by the Sisterhood of Karn.

The Flame was fuelled by an underground natural gas pocket that escaped through a fissure in the rock, acting like a natural gas nozzle.

The Elixir was created from water from an underground spring. It passed through the rocks heated by the Sacred Flame directly below them. The rocks contained rare minerals and compounds that were released into the water when heated by the Flame.

Over countless millennia, carbon and soot had clogged the nozzle. The flame had been reduced to barely more than a inch high, resulting in almost no generation of the Elixir. The Fourth Doctor solved the problem by placing a firecracker wrapped in blue touch paper in the fissure, blasting the soot loose and re-igniting the Flame. (DW: The Brain of Morbius)