In the era before VCRs, Howard Hughes would call the Las Vegas TV station he owned and demand they run this particular movie. Hughes so loved this film that it aired on his Las Vegas station over 100 times during his lifetime.
Ernest Borgnine served in US Navy in from 1935 to 1945. Rock Hudson & Gerald O'Laughlin served in the US Navy & Marine Corps, respectively, during WWII.
In one scene Patrick McGoohan was supposed to dive into the flooded torpedo room of the nuclear sub to rescue a trapped naval officer. Being a strong swimmer, he insisted on doing the scene himself rather than use a stuntman. A change was made to the script so allowing Olympic swimming champion Murray Rose, who'd been cast in another role, to do the scene with him in case anything happened. It was only after the scene was completed that Rose revealed that whilst he and McGoohan were standing up to their necks in the rising water just before the cameras rolled, Pat had whispered to him "Now I've done it, my foot's stuck". Rose dived down and freed his foot which had become wedged tight in the torpedo rack.
Unique and innovative underwater camera equipment was developed for this movie by 2nd unit cameraman and cinematographer 'John M Stephens', a former U.S.A. Navy diver, who is billed in the credits for additional Arctic photography. The camera system enabled the first ever filming of a continuous submarine dive and this technical innovation produced some outstanding photography for the picture. This achievement was encapsulated in an accompanying MGM short promo film The Man Who Makes the Difference (1968) which is available on the DVD for this movie.