Mole

A mole was a small Earth mammal. They were famous for their ability to burrow holes. On one occasion, a mole near Kent quickly dug a number of holes underneath some liquid metal that Daleks had laid down. This destabilised the structure and caused the liquid metal sprayer to entrap most of the Daleks in the metal. As a result, the Dalek invasion was thwarted. (PROSE: The Small Defender)