||This article needs additional citations for verification. (February 2010)|
In the classic 1956 science fiction film, "Forbidden Planet", the extinct race of advanced beings of the planet Altair IV are known as the "Krell". The Krell had reached a stage of technological and scientific development so advanced that they were able to construct a machine with virtually unlimited power, a machine that turned their thoughts into reality.
Another Krell device that played a prominent role in the movie was their educator, a device that operated directly on the brain to measure intelligence and impart knowledge. It was used by Dr. Morbius to enhance his intelligence and begin learning the knowledge of the Krell. When Lt. "Doc" Ostrow tried it, it imparted the knowledge of what happened to the Krell, but also caused fatal injury to his brain.
At the opening of the film, the lone researcher studying the extinct Krell, Dr. Morbius, does not know exactly what happened to the Krell; nor does he know what they looked like. No record of their physical nature has survived. Except in the form of their characteristic arch, the doorway they used to move between rooms. The doorway, much wider at the middle than at the top and bottom, suggests a being of enormous girth.
Dr. Morbius has discovered that in a single day and night, this entire race disappeared with no trace remaining above ground (likely a nod to Plato's tale of Atlantis). Later in the film we learn that the Krell's 8,000-cubic-mile (33,000 km3) machine was so advanced that it gave physical form and life to their Id. For the advanced Krell, this Freudian personality characteristic was long forgotten, yet not eliminated; combined with the power of their machine, the unbridled emotions of their Ids were able to eradicate the entire Krell race. Thus two thousand centuries before the film is set, the Krell had become extinct.
In popular culture 
Reference: 1979 Cinefantastique Magazine Double-Issue (Volume 8 - Number 2 & Volume 8 - Number 3) MAKING FORBIDDEN PLANET - By Frederick S. Clarke and Steve Rubin
The steps leading up to the Krell door ... were designed to be separated by a smooth ramp in the middle.
In the article the cinematographer of the Forbidden Planet film, George Folsey states:
"The Krell were originally frog-like in nature with two long legs and a big tail. They were never shown, but it was indicated in the original screenplay that the ramps between the steps were designed to accommodate their dragging tail."
- Wilson, Bill. "The Short GKrellM FAQ". "If you have seen the movie Forbidden Planet, you might recall the Krell had a room with wall to wall meters for monitoring their power systems, and that is what I was thinking of when I came up with the GKrellM name."