Lava
From Tardis Wiki, the free Doctor Who reference
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)
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)
