Lava

Lava was molten rock, often released by volcanic eruptions.

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

In 1st century Pompeii, an energy converter could take lava from the Vesuvius and use the power to create a fusion matrix, which welded Pyrovile to human. (TV: The Fires of Pompeii)

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

Behind the scenes
In the webcast Scream of the Shalka, which this wiki considers to be an invalid source, the Shalka, a race made almost entirely out of lava, attempted to invade Earth in 2003.