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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 vast underground machine with virtually unlimited power; a machine that could turn thoughts into reality and project that reality anywhere on the planet.
Another Krell device that played a prominent role in the movie was their plastic educator, a device able to create a three-dimensional visualization of the operator's thoughts while acting directly on the brain to measure intelligence and impart knowledge. Although previously demonstrated fatal to humans, its use by Dr. Morbius had boosted his IQ such that he could understand some of the basic science of the Krell (allowing him, for instance, to "tinker together" the amazing Robby the Robot.) When Lt. "Doc" Ostrow tried it he gained the ability to reason out and tell Commander J.J. Adams what had caused the downfall of the Krell -- "Monsters from the Id"—but it also caused fatal injury to his brain.
At the opening of the film Dr. Morbius, the lone researcher studying the extinct Krell, does not know exactly what happened to them; 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 used to move between rooms. This doorway, much wider at the middle than at the top and bottom, suggests a being of enormous girth.
Morbius has discovered that in a single day and night, over 2,000 centuries past, the entire Krell race disappeared and in the time since all above-ground evidence of their civilization has disappeared (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. Even for the advanced Krell this Freudian personality characteristic, although long forgotten, had not been eliminated. When combined with the power of their machine, the unbridled emotions of their Ids were all at once let loose to eradicate the entire Krell race.
In popular culture
The Department of Energy contractor, The Krell Institute, was also named after the movie. 
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."
- "Inside Krell". Retrieved 2013-06-07. "Krell is named after the advanced civilization that once inhabited the planet Altair IV in the classic 1956 science fiction movie “Forbidden Planet.”"