Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Communication with submarines is difficult because radio waves don't travel well through thick electrical conductors like salt water. The obvious solution is to surface and raise an antenna above the water, then use ordinary radio transmissions. Early submarines had to frequently surface anyway for the oxygen needed by their diesel engines. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. In order to allow for communication with submerged submarines, several techniques are used.

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