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)

In a dream state, Clara Oswald threw the keys to the TARDIS into lava, destroying them to her mind, while attempting to persuade the Twelfth Doctor to help her following the death of Danny Pink. (TV: Dark Water)

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.