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Sonar, an acronym for sound navigation and ranging, is a system that uses sound waves to detect and locate objects underwater.

The idea of using sound to determine the depth of a lake or ocean was first proposed in the early nineteenth century. Interest in this technique, called underwater ranging, was renewed in 1912 when the luxury sailing vessel Titanic collided with an iceberg and sank. Two years later, during World War I (1914–18), a single German submarine sank three British cruisers carrying more than 1,200 men. In response, the British government funded a massive effort to create an underwater detection system.

The entire operation was conducted in complete secrecy, but the first working model was not ready until after the war ended. The project operated under the code name...