In the mid-1950's, during the Cold War, the US Navy installed an underwater surveillance system to track submarines. The Sound Surveillance System (SOSUS) is a multibillion-dollar network of hydrophone arrays mounted on the seafloor throughout the Atlantic and Pacific oceans. The SOSUS system takes advantage of the sound channel that exists in the ocean, which allows low-frequency sound to travel great distances. This channel is called the SOund Fixing And Ranging, or SOFAR, channel (See How does sound travel long distances? The SOFAR Channel). Low-frequency sound generated by submarines can be detected at long ranges by hydrophone arrays located on continental slopes and seamounts, and connected by undersea cables to onshore facilities. These hydrophone arrays listen to the ocean, record sounds, and transmit the data back to shore stations for analysis.
 
       SOSUS consists of bottom mounted hydrophone arrays connected by undersea communication cables to facilities on shore, such as a remote Naval Facility (NAVFAC) and a Naval Ocean Processing Facility and Meteorology Center (NOPF/MEC). Photo Courtesy of NOAA Ocean Explorer.

       At the end of the Cold War, the Navy decided to allow this system to be used by scientists with suitable security clearances, in what was called "dual-use." SOSUS is now used to study hydrothermal vents and pinpoint underwater volcanic eruptions. The system is also used to study the vocalizations of whales. Scientists can study and track whales in the Atlantic and Pacific oceans using the SOSUS hydrophone arrays. This system has also been used to measure ocean temperatures in relation to climate change. By measuring the travel time of sound waves, the SOSUS system is able record average ocean temperature changes over an ocean basin.

       For more detail on the history of SOSUS, please visit the section on the history of underwater acoustics.