: The film’s pioneering underwater sequences are widely credited as a major inspiration for Ian Fleming’s James Bond novel (and later film), Thunderball . Key Themes

: Unusually for war films of the era, it avoids heavy jingoism, portraying the Italian divers not as fanatics but as skilled, professional adversaries performing dangerous work.

: The film is based on Marshall Pugh’s 1956 book, Commander Crabb . It was produced just two years after the real Lionel Crabb vanished under mysterious circumstances during a Cold War reconnaissance mission for MI6 in 1956.