Cathonian

Cathonians were a race of mole-like people native to the planet Cathon. In the distant past, their people were embroiled in a great war that resulted in the creation of cybernetically enhanced Cathonians known as Silex.

When the Silex revolted against their creators, they poisoned Cathon's atmosphere, forcing the Cathonians to flee underground. Their society eventually began to revolve around the large drilling vehicles they used to navigate the planet's brittle crust, which would consume various ores that could be fed into the machines' furnaces to keep them running. The largest of these vehicles were called drill towns, which soon became the prime targets of the Silex when they pursued the Cathonians underground in order to harvest their people for conversion. Over the years, the number of drill towns dwindled as they were claimed by the Silex. One of the last of these drill towns, the Vermes, was sold out to the Silex when Lucretia Bell, wife of the ship's chief steering officer, arranged for its capture in exchange for becoming their new queen.

Thanks to the intervention of the Fourth Doctor and Romana II, however, Lucretia and the rest of the Silex were defeated. Upon the Silex's defeat, the Doctor and his allies use the Silex drill, the Annelid, to return to the surface where the find that the air had long since become breathable, allowing the Cathonians to begin rebuilding their civilisation. (AUDIO: Subterranea)