When Morbius has taken a mortal wound from his monster and destroyed his evil self, he realizes that mankind is not ready for the power available to him through the Krell technology. Without explaining what he’s doing, Morbius instructs Adams to turn “that disc”. Adams mindlessly obeys, and a plunger emerges from the floor near him.
Adams initiates the irreversible Krell self destruct mechanism.
Morbius commands Adams, “The switch, throw it.” Again, Adams does as he’s told, and the plunger clicks into place as a red ring (the same red ring below the educator lever) illuminates. Then, and only then, Morbius explains that, “In 24 hours you must be 100 million miles out in space. The Krell furnaces’ chain reaction… they cannot be reversed.” You think that with that kind of finality, he might have bothered to explain what was going to happen, or inquire whether the crew could make it out that far in that amount of time, but you know, science knows best.
The krell self-destruct warning signal is a silent, blinking red ring around the plunger.
Adding insult to injury, the complete warning system for this massive, solar-system-sized explosion consists of, in total, a silently pulsing red ring around the base of a plunger located in the heart of a hidden underground city behind a series of impenetrable doors sealed with combination locks. There is no klaxon, no lights seen elsewhere to indicate that your star system is about to go boom. I guess if you didn’t know, you didn’t really need to know.