Sacred Flame

The Sacred Flame is located on the Planet Karn, and is guarded by the Sisterhood of Karn. It is the heat source that creates the life-prolonging Elixir of Life.

The Flame is fueled by an underground natural gas pocket, which escapes through a fissure in the rock, acting like a natural gas nozzle.

The Elixir is formed as water from underground springs pass through the rocks heated by the Sacred Flame directly below them. The rocks contain numerous rare minerals and compounds which are released into the water when heated by the Flame.

Over the course of countless millenia, carbon and soot had clogged the nozzle so much that 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 in the fissure, blasting the soot loose and re-igniting the Flame. (DW: The Brain of Morbius)