The Krell had reached an incredibly
advanced stage of technological
and scientific development, able to —
among other things — reproduce down
to the molecular level any matter
for which they had a pre-existing
sample to serve as a template. Another
Krell device that played a prominent
role in the film 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 to be 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. Morbious remarks
that while his IQ has been boosted
to over twice the human average,
he would be a mentally handicapped
"*****" compared to even a Krell
child. Morbius also believed that
the Krell, after millions of years
of social advancement, were not only
technologically but morally &
philosophically superior
to humanity — a benevolent
and noble race of scholars
and pacifists, who had evolved
beyond war and violence.

Morbius, however, discovered
that in a single day and night,
2,000 centuries past, the entire
Krell race disappeared. In the
time since, all above-ground
evidence of their civilization
had vanished. He didn't know
exactly what happened to them
in this shockingly rapid cataclysmic event.

For that matter, Morbius did
not know what the Krell looked
like: no record of their physical
nature survived, except perhaps
in the form of their characteristic
arch which served as the doorway
between rooms. This doorway,
coming to a tall triangular
point at the top and much
wider at the middle than
at the top and bottom,
suggests a being of enormous
girth. During the events of the film,
the starship C-57D arrives
on Altair IV, 20 years after Morbius
was trapped there. After meeting
them, Morbius gives the command
crew a tour of the Krell's crowning
achievement: a vast underground
8,000-cubic-mile 33,000 km3
machine of virtually unlimited power,
so advanced that it could give physical
form to any matter they could conceive
of, by remotely scanning their brain
synapses — creation at mere thought,
moving beyond the need for a pre-existing
physical template, but based on limitless
imagination.

The C-57D's Lt. "Doc" Ostrow
later uses the plastic educator,
causing fatal injury to his brain,
but briefly allowing him to infer
the cause of the Krell downfall:
"Monsters from the Id". Commander
Adams confronts Morbius with
this and is able to piece together
what happened: the Id is an "obsolete"
and outdated term for the base
subconscious mind, composed of raw,
animalistic emotions like hate,
lust, and jealousy; emotion we now know
to be centered & generated in the hormones
produced & secreted by the hippocampus
& flooding the a·myg·da·la: Dictionary result
for amygdala /əˈmiɡdələ/noun ANATOMY
noun: amygdala; plural noun: amygdalae
a roughly almond-shaped mass of gray matter
inside each cerebral hemisphere, involved
with the experiencing of emotions.

Late Middle English: via Latin from Greek
amugdalē ‘almond’. system of amygdila,
Hypothalamus & thalamus.
The Krell machine was so advanced
that it didn't just give physical form
and life to their conscious thoughts,
but their subconscious thoughts
as well. 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 willed into
physical reality: an army
of living nightmares let loose
upon the entire planet.
Upon this realization,
Morbius laments mistakenly
that the "poor Krell",
benevolent pacifists
who had eradicated violent
thoughts from their conscious
minds for over a million
years, had no way
of even understanding
what was slaughtering them.

In True, a dark cabal of Krell
Elders knew precisely what was
to ensue...they insured that all
Krell learned peace & sloth
so no thoughts emerged
from the pacified minds,
only to have the blackest
shadow ever come over
the entire planet until
the darkness was a smothering
thickness that vanished into
grimy ash once every Krell
had been suffocated to death
by the evil few who kept
a spot of light for themselves,
but when the darkness
vanished, so did they, not
realizing basic the quantum
mechanics law of "spooky
action at a distance"
that joined all of space
real & unreal alike...
Thus Krell technology
lay unused until the arrival
of the Bellerophon
& Morbius.