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