Trivia

It was noted that certain plot elements bore a striking similarity to plot elements from Polish writer Stanislaw Lem's (at the time) recent 1961 novel "Solaris." That novel would later be the basis of authorized films including a TV-Movie (Solaris (1968)) in 1968, a cinema version by Andrei Tarkovsky in 1972 (Solaris (1972)) (both from the Soviet Union) and an American production (Solaris (2002)) in 2002.

The Earth is run by the UN and there are no more wars. The date on the letter John Agar reads is 10th September, 2001 one day before 9/11.

When officials at American International viewed the completed film, they decided that some of the Danish-produced special effects were so poor that they needed to be replaced. Two members of the independent special effects company Project Unlimited, Jim Danforth and Wah Chang, shot new footage to replace some of the Danish special effects. Some of the deleted footage was also replaced with tinted black-and-white monster footage from The Spider (1958).

Among the replacement footage used in the American International version was a brief special effects shot from The Angry Red Planet (1959).

The major contribution made to the American International version by Jim Danforth and Wah Chang was the giant cyclopean rodent monster. The monster's roar was actually Rodan's roar taken from Rodan (1956) (US title: "Rodan").