Giacomo Rossi-Stuart, who plays the key featured role of Rosati, is dubbed By 'Robert Rietty (I)'.
According to the book 'Brassey's Guide to War Films' by Alun Evans, during this movie's theatrical season, there were " . . . strong (and never disproved) rumours at the time of its release that the film's real-life hero, Commander Crabb See: Lionel Crabb, had mysteriously died while diving to spy on a Soviet warship visiting Britain."
According to the book "A Biography Of Sid James", James was offered the role of Chief Petty Officer Thorpe on the recommendation of his friend Laurence Harvey, but their friendship ended during the filming. He found Harvey to be "pompous and full of his own importance". It's said that the two never spoke to each other again after filming had ended.
Several Avro Shackletons (which did not enter service with the Royal Air Force until 1951) are at the forefront of the scene where Crabb (Harvey) is seen defusing an Italian mine which had just been brought ashore.
This film represents an attack on the vessel the "Olterra" but in real-life events there was no such attack.
This movie went into production about a year after the much publicized disappearance of the movie's real life lead character, Lionel "Buster" Crabb OBE, GM (born 28 January 1909). Lionel Crabb was presumed dead on 19 April 1956 due to an alleged encounter from the Cold War.
This movie's dedication states: "This film is dedicated to the frogmen of all nations who have died beneath the sea and in particular to the memory of Commander Lionel Crabb, G.M., R.N.V.R." See: Lionel Crabb.
This movie's opening credits state: "The underwater sequences in this film were photographed beneath the Mediterranean."