Silurians

Intelligent reptilian life form which ruled the Earth during the Cretaceous (136 to 65 million years BC), and not the Silurian (430 to 395 million years BC).  They and their underwater "cousins", the so-called  "Sea Devils". were eventually driven to hibernate in underground shelters by some undetermined cosmic event.   Theories about the nature of this event included: a wandering planetoid causing a change in the Van Allen Belt; movements of the moon; departure of Earth's twin planet, Mondas; or possibly the quasi-psychic awareness of the forthcoming crash of a space liner from the future.

The Silurians of Wenley Moor were reawakened from suspended animation in 1971 because of power leaks from the neighboring British atomic research center.  The Old Silurian met Dr. Quinn, then the Third Doctor, and became convinced that his people could share Earth peacefully with mankind.  However, the Young Silurian tried to wipe out mankind with a  deadly virus.  After that had failed, he and the Silurian scientist, Icthar, attempted to use a molecular disperser to destroy the Van Allen Belt, and thus render Earth inhabitable for humans.  Thwarted by the Third Doctor, they returned to suspended animation.  Their shelter was then blown up by UNIT.

Icthar survived, and reawakened in 2084.  HE reanimated a Sea Devil   commando, led by Sauvix, and attacked Sea Base Four.  His plan was to launch nuclear missiles to provoke a war between the two blocs, and thus eradicate mankind (which had been the Young Silurian's last wish).  His efforts were defeated by the Fifth Doctor, who used deadly hexachromite gas to wipe out the Silurians.