Communication with submarines is difficult because radio waves do not 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 surface frequently for oxygen needed by their diesel engines to charge their batteries. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months. To communicate with submerged submarines several techniques are used.
Read more about Communication With Submarines: Acoustic Transmission, Very Low Frequency, Extremely Low Frequency, Standard Radio Technology
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“The present war having so long cut off all communication with Great-Britain, we are not able to make a fair estimate of the state of science in that country. The spirit in which she wages war is the only sample before our eyes, and that does not seem the legitimate offspring either of science or of civilization.”
—Thomas Jefferson (1743–1826)