Cathon

Cathon was a planet inhabited by mole-like people that was ravaged by war in the distant past. This war ultimately resulted in one of the sides cybernetically enhancing their soldiers, creating what would be come to known the Silex. When the time came to deactivate the Silex, they revolted against their creators and poisoned the air of the surface, forcing them to flee underground. The Cathonians constructed large drilling vehicles in order to navigate the through the planet's brittle crust. These vehicles sustained themselves by constantly consuming various ores that could be fed into their furnaces to keep them running and could be as big as a house or the size of small town. Eventually the Silex pursued the Cathonians underground and systematically hunted down all the drill towns, harvesting their people and converting them into Silex. Over the years the number of drill towns dwindled until only a handful remained unclaimed by the Silex. The Silex were ultimately defeated by the Fourth Doctor and Romana II, and shortly after it was discovered that the poison on the surface had long since cleared up, allowing the Cathonians to start rebuilding their civilisation.