Factual error: At the end of the movie the cast is counting down to the explosion of the Krell furnaces. They reach zero and the furnaces explode. They see the light from the explosion just it happens. But they are 100 million miles out by then. The light would have taken over 8 minutes, about as long as light from the sun takes to reach earth, to reach the ship.
Factual error: The Doctor wonders how the creature can survive three billion electron volts. This is about 0.0000000006 joules. A baseball traveling a 2 miles per hour is one joule. It would take a lot more than three billion electron volts. It would take about 60 billion billion electron volts just to supply one joule. The energy hitting the monster was less than that supplied by running into a gnat.
Answer: Many of the sound effects use an electictronic instrument called a theremin which was invented in 1919. You may hear it used more musically in The Beach Boys song "Good Vibrations." The visual effects would have been animated composites as used in Star Wars and Ghostbusters.
Myridon