Trivia: Patrick McGoohan committed himself to appearing in "Ice Station Zebra" whilst he was filming his famous TV series "The Prisoner". However, as is well known, "The Prisoner" went massively overbudget and seriously behind schedule. As a result, McGoohan had to leave filming of "The Prisoner" in late 1967 for several weeks, in order to appear in "Ice Station Zebra". This is the reason why several episodes of "The Prisoner" have McGoohan either absent or only in previously filmed flashbacks and inserts...McGoohan could not be filming both "The Prisoner" at Elstree, and "Ice Station Zebra" in Hollywood at the same time.
Trivia: The submarine used in the movie as a (supposed) nuclear submarine was the U.S.S. Ronquil (SS-396), a WWII Balao-class diesel-electric submarine that had received a GUPPY IIA conversion. The number painted on the sail (509) was selected because it was from a block of hull numbers canceled at the end of WWII and never used.