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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“We ought to celebrate this hour by expressions of manly joy. Not thanks, not prayer seem quite the highest or truest name for our communication with the infinite,—but glad and conspiring reception,—reception that becomes giving in its turn, as the receiver is only the All-Giver in part and infancy.”
—Ralph Waldo Emerson (1803–1882)