Submarine
A submarine was a type of specialized watercraft that could operate underwater, which was employed by several Earth navies and other operators.

Deep Flight 1 was a 20th century submarine on Earth. (, opening credits)

Submarines also existed in the mirror universe. Among them were Type VII submarines and Project 705. ()

Malcolm Reed's great-uncle was a member of the submarine arm of the Royal Navy. As chief engineer he served aboard , a nuclear powered submarine. Reed's great-uncle died after the submarine struck a mine, giving his life to help the rest of the crew reach the escape pods by keeping the reactor online long enough. ()

In the Julian Bashir, Secret Agent holoprogram, a submarine had at one point waited for the secret agent near Iceland. ()

Upon seeing Monean research vessels break through the surface of The Waters and travel through space, Tom Paris wondered in 2375 whether those vessels were "starships or submarines." ()

Appendices

Background information 
In , a Romulan Bird-of-Prey, equipped with a cloaking device, metaphorically represents a submarine, hiding from and combating the , which represents an opposing vessel on the surface of a stretch of water. In effect, two submarine war movies,  and , served as the basis for the episode's plot. Also, the cramped bridge of the Bird-of-Prey was deliberately evocative of a submarine, and it even included a periscope-like scope. (The Star Trek Compendium, 4th ed., p. 40)

The sets of the  and  were designed to be reminiscent of submarine interiors. In the former's case, one episode in particular, , was intentionally reminiscent of well-known submarine movies such as .

The images from the opening credits of "In a Mirror, Darkly" were recycled footage from the 1990 Paramount Pictures espionage movie  (the studio models for which, including that of the , built by Gregory Jein, Inc., subcontracted by Boss Film Studios. ), and the 2000 Universal Studios war movie ; depicted were a Soviet Alfa-class, aka Project 705, and a German  submarine, the most ubiquitous type used by the German Kriegsmarine in World War II.

External link 
 

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Earth vehicles