The Gelth were blue gaseous life-forms. They claimed to have lost their corporeal forms as a consequence of the Last Great Time War, though later actions by the Gelth put the truth of this statement in doubt. They arrived on Earth via the Spacetime Rift at an undertaker's house in Cardiff in 1869. Their forms could not be maintained in Earth's atmosphere without suspension in a gaseous medium, so they inhabited the gas pipes common to Victorian era households. Further, though, they also would take possession of recently deceased corpses. When possessing corpses, they look much like ordinary humans (provided that the corpse has yet to enter the autolytic stage of decomposition), with only two fundamental differences: their irises vanish or turn white, and blue veins are clearly visible on their ghastly pale skin. Gelth make an unearthly shrieking noise for an unknown reason, particularly when they've possessed someone.