Communication with submarines is a field within military communication that presents technical challenges and requires specialized technology. Because radio waves do not travel well through good electrical conductors like salt water, submerged submarines are cut off from radio communication with their command authorities at ordinary radio frequencies.
Submarines can surface and raise an antenna above the sea level, then use ordinary radio transmissions, however this makes them vulnerable to detection by anti-submarine warfare forces.
During the cold war, however, nuclear powered submarines were developed that could stay submerged for months. In the event of a nuclear war, submerged ballistic missile submarines have to be ordered quickly to launch their missiles.
So as to effectively and anonimously communicate with submarines, nations have developed very low frequency radio waves that can penetrate seawater a few hundred feet. A few nations among them India have developed land based transmitters for very low frequency radio waves.
Today, Defense Decode, posted a photo of an Indian Navy's very low frequency radio waves transmitting station. The photo was followed by a statement that read, "The Indian Navy's state-of-the-art very low frequency transmitting station to communicate with with nuclear submarines, at INSKattabomman. The facility can communicate with submarines, which have trailing wire antenna to pick up the coded very low frequency radio waves, on an uninterrupted basis throughout the year."