In 1901, a Boston-based firm called the Submarine Signal Company began manufacturing a system of communications tools that exploited this property of aquatic sound waves: underwater bells that chimed at regular intervals, and microphones specially designed for underwater reception called “hydrophones.” The SSC established more than a hundred stations around the world at particularly treacherous harbors or channels, where the underwater bells would warn vessels, equipped with the company’s hydrophones, that steered too close to the rocks or shoals.
