Lava

Lava was molten rock that came out of an opening in the surface of a planet or moon.

On the parallel Earth visited by the Third Doctor, the Inferno Project had progressed far enough to release lava. (TV: Inferno)

The Shalka, a race made almost entirely out of lava, attempted to invade Earth in 2003. (WC: Scream of the Shalka)

Florinall 9 was a planet known for its lavafalls or, as the Eleventh Doctor called them, firefalls. (WC: Pond Life)