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The noises made by the dolphins - eventually becoming recognisable speech - were provided on the soundtrack by the film's scriptwriter, Buck Henry (who was uncredited in this role).
Lamar Boren was responsible for filming all underwater scenes.
The dolphins who appeared in this film were trained by Peter Moss, Kathy Trout and Erica Abt.
Dr. Jake Terrell: [on returning to the island]: I go away for one lousy afternoon and the whole place falls apart!
Dr. Jake Terrell: [discussing dolphins]: Why, as mammals, were they compelled - or did they decide - to leave the land and return to the sea?
Harold de Milo: Is this blackmail, Mr. Mahoney?
Curtis Mahoney: Goodness, no!
This film, like Robert Merle's original novel, was inspired by the researches of Dr. John Lilley. He threatened the makers of the film with a lawsuit, although it came to nothing (perhaps because the film was a box-office flop).