Name one primary means of establishing communication with submarines.

Extremely Low Frequency (ELF) communications is a primary means of establishing communication with submarines due to the unique characteristics of ELF waves. These waves can penetrate water to significant depths, allowing them to reach submarines that are operating far below the surface. The ability of ELF waves to travel long distances without attenuation provides the crucial capability to communicate with submerged vessels where other forms of communication, such as direct radio transmission, are not feasible due to the water's shielding properties.

While other methods of communication, such as direct radio transmission and sonic wave signals, have their applications, they face limitations when it comes to communicating with submarines that are deep underwater. Direct radio transmissions are typically effective only at periscope depth or above, and sonic wave signals are limited to short distances and specific operational scenarios. Visual signals are also not viable at depths below the surface. Therefore, ELF communications stands out as the most reliable and functional method for maintaining communication with submarines while they are submerged.