The sound effects of the rocket control room are reused from Star Trek TOS original pilot and first season 2 part episodes for the bridge of USS Enterprise.
The opening shot of the story (with title: "An Island In The Adriatic Sea") actually depicts Castillo San Felipe del Morro, the historic fort which guarded the harbor entrance of San Juan, Puerto Rico.
The helicopter seen is a Fairchild-Hiller FH-1100, FAA registry number N9055 (though the N is blacked out to conceal its U.S. registration). The same helicopter, painted differently, was used in The Hostage (1970)
Leonard Nimoy and Barry Atwater also acted together in the Star Trek original series show "The Savage Curtain". Atwater played the role of Surak, the founder of the Vulcan way.
Location filming was shot at Fort MacArthur in San Pedro, Los Angeles, California. It was formally created on October 31, 1914, and named for Lieutenant General Arthur MacArthur.