Lava
From Tardis Wiki, the free Doctor Who reference
Lava was molten rock, often released by volcanic eruptions.
A TARDIS key could be melted and destroyed only by lava. (TV: Dark Water)
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.
