Throxian Lavaworms were a genetically-engineered species designed to increase the temperature of Throx. They bred under the planet's surface and caused its destruction, after which they were wiped out by the Time Lords.
Mother Finsey placed Lavaworms beneath the surface of Ephros to cause its destruction. However, the Third Doctor was able to transmat them off of the world and into space. (AUDIO: The Transcendence of Ephros)