The Doctor first met Silurians in the episode Doctor Who and the Silurians, (1970). Long before Humans evolved, Silurian astronomers detected a small planet on a collision course with Earth. They went into underground hibernation to survive the disaster. However, this small planet was in fact the Moon, coming into orbit with the Earth; the disaster never happened, and due to a fault in the hibernation systems, they never woke up. Various tribes have occasionally woken up since. Scientifically advanced, they have complex cities hidden far underground. Throughout their meetings with the Doctor, the Silurians have evolved many times. Various types have a third eye that glows when they talk, and other Silurians have large shell-like armour, or human-like facial features.