A submarine’s military effectiveness has always depended on its ability to remain submerged and undetected. Surprisingly, the earliest vessels operated blindly under the sea, and until the twentieth century the only way for one to see was by surfacing. Of course, this revealed the submarine’s location to nearby vessels and conceded its invisibility. Since 1903, however, naval submarines have used a tubular optical instrument known as a periscope to get a look at their surroundings. To do so, they would ascend to approximately 60 feet below the surface, deploy their periscope, assess the field of view, and then quickly descend beyond the reach of surface vessels. While at periscope depth, the submarine surrendered some maneuverability and tactical ability for the sea level surface information. This was a well understood trade-off.

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