Krell

The Krell are the extinct race of advanced subterranean beings of the planet Altair IV in Forbidden Planet. A human called Dr. Morbius was the only researcher to ever study the extinct Krell.

The Krell had reached an incredibly advanced stage of technological and scientific development, able to reproduce down to the molecular level any matter. Dr. Morbius used their "plastic educator" to boost his IQ such that he could understand some of the basic science of the Krell. Morbius believed that the Krell, after millions of years of social advancement, were not only technologically but morally superior to humanity—a benevolent and noble race of scholars and pacifists, who had evolved beyond war and violence.

Morbius eventually discovered that in a single day and night, 2,000 centuries ago, the entire Krell race disappeared. All above-ground evidence of their civilization had vanished and he didn't know exactly what happened to them in this shockingly rapid cataclysmic event.