Lava
From Tardis Wiki, the free Doctor Who reference
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)
