A submarine was a specialized watercraft that could operate underwater and which was employed by most Earth navies.
Prior and during the World War II, Germany was one of the world's leading nations in the area of submarine technology. Their submarines were named "U-Boats", the term deriving from the German Navy's system of naming its submarines with U- followed by a number, where the "U" stood for Unterseeboot (literally, "undersea boat"), the German word for submarine.
Malcolm Reed's great-uncle once served on a submarine in the Royal Navy. Reed's uncle died while trying to stop on-board flooding after the submarine struck a mine. (ENT: "Minefield")
The sets of the USS Defiant and the Enterprise NX-01 were designed to be reminiscent of submarines interiors. In the former's case, one episode in particular, DS9: "Starship Down", was intentionally reminiscent of well-known submarine movies such as Das Boot.
The images from "In a Mirror, Darkly" were recycled footage from the 2000 Universal Studios war movie U-571; shown were German Type VII submarines, the most ubiquitous type used by the German Kriegsmarine in World War II. While the titular U-boat had a historical counterpart, the events depicted in the movie were actually loosely based on the British capture of U-110 and her Enigma coding machine and cipher keys.