The Sea Devils where the original reptilian inhabitants of Earth. Intelligent and civilised, they were the marine equivalent of the Silurians. Their people dominated the Earth at the end of the dinosaur era, just as mammalian life was beginning during the Cretaceous (136 to 65 million years BC), and not the Silurian (430 to 395 million years BC).

They, and their "cousins" the Silurians, were eventually driven to hibernate in underground shelters when their scientists detected a large asteroid approaching Earth, reasoning that as it passed, it would draw the atmosphere away, making Earth uninhabitable.

The reptile people built themselves vast underground and undersea shelters, placing themselves into hibernation, setting timing mechanisms that would awaken them once the atmosphere returned after the asteroid had gone. However, the asteroid fell into orbit, becoming Earth's Moon, so the atmosphere never altered and the timing mechanisms never awoke them. Over millions of years, Earth's continents moved, destroying all evidence that their civilisation had ever existed.

One Sea Devil shelter was disturbed off the south coast of England, when a commando of Sea Devils awoke from suspended animation in 1973 and began sinking ships in the British Channel. They were

contacted by the Master who offered to ally himself with them.

The Third Doctor and the Chief Sea Devil talked peace, but they were thwarted by the interference of the Master and Walker, a civil servant who ordered a depth charge attack. In the end, the Doctor was forced to engineer an explosion which destroyed the Sea Devils’ undersea base before any more Sea Devils could be awakened (LLL).

The Silurian Icthar reanimated a Sea Devil commando led by Sauvix in 2084 AD, and used them to attack Sea Base Four. His plan was to launch nuclear missiles to provoke a war between the two blocs, and thus eradicate mankind.

His efforts were defeated by the Fifth Doctor, who used deadly hexachromite gas to wipe out the Silurians and the Sea Devils (6L). ...... Also see Silurians.