Silurians should more accurately be described as Cretaceans, although their first given name has stuck. They once ruled the Earth at the time of the dinosaurs, when apes were considered as unintelligent playthings. The impending disaster of a planet crashing into the Earth forced the Silurians to retreat into underground caverns, where they went into hibernation. The disaster, however, never happened: the planet went into orbit round the Earth and became known as the Moon. The Silurians' instruments never detected the post-disaster conditions they expected, forcing the Silurians into a sleep of many thousands of years.

There are colonies of Silurians all over the Earth, but the creatures awaken only when man's scientific experiments unleash enough energy to activate their hibernation machinery. Silurians are dismayed that man, a mere ape, is the dominant life-form; the Silurians would like to see the upstart eliminated so their golden civilization could be restored.

Silurians have manlike brown bodies that are covered in fine scales; their splayed feet have three toes and their three-fingered hands end in talons. Flat trapezoid ears jut out from the side of their heads, and the top of their heads are ridged and crested. A collar of rough skin falls down to their shoulders. A third eye, located in the central crest, emits infra-red radiation that can be used as a weapon or emitted in pulses to operate scientific machinery, electronic locks and the like. The small, round and almost sucker-like mouth is not suited to human speech, resulting in a deep, warbling voice.

The Silurians are related to the Sea Devils, their underwater counterparts who were also forced into hibernation.

Silurians are vulnerable to hexachromite gas, which is poisonous to them.