Lava
From Memory Alpha, the free Star Trek reference
Lava is molten rock that is exuded from the ground, such as during a volcanic eruption.
Commander Kruge was kicked into a sea of lava while fighting with Captain Kirk on the Genesis planet. (Star Trek III: The Search for Spock)
In a story told by Kor, he, Kang and Koloth battled T'nag between "walls of fire" and "rivers of lava". (DS9: "The Sword of Kahless")
In the Julian Bashir, Secret Agent holoprogram, Hippocrates Noah attempted to release "millions of tons" of lava from beneath Earth's tectonic plates, causing the surface of the planet to subside and be flooded by the oceans. (DS9: "Our Man Bashir")
