This movie was based on Larry Forrester's second "Fathom' novel, which was never published and was only in draft form when this movie was made. Forrester's first "Fathom" novel was called "A Girl Called Fathom", published in 1967.
Peter Merriwether's (Anthony Francoisa's) convertible sports car is a 1965 Ferrari 275 GTS.
Fathom's name was an amalgam of the first names of the character's uncles who were called (in order of the letters in Fathom's name) Freddie, Arthur, Tom, Harry, Oscar, and Milton.