SOSUS, an acronym for SOund SUrveillance System, is a chain of underwater listening posts located for the most part across the northern Atlantic Ocean near Greenland, Iceland and the United Kingdom -- the so-called GIUK gap. It is operated by the U.S. Navy originally with the purpose of tracking Soviet submarines, which would have had to pass through the gap in order to attack shipping in the Atlantic. A selection of sites in other locations in the Atlantic and Pacific Ocean have also had SOSUS stations installed.

SOSUS development was started by the Committee for Undersea Warfare in 1949, a panel formed by the Navy, in order to further research into anti-submarine warfare. At the time the main concern was snorkeling diesel submarines, and the panel quickly decided that the solution was to use low-frequency sound detectors which would be able to hear the sound of their engines from hundreds of kilometres. Each site would consist of several detectors, allowing them to triangulate the position of the submarine. They recommended that $10 million be spent annually to develop such systems

In 1950 the committee's report was used to set up Project Hartwell at MIT, named for the director of the committee, Dr. G.P. Hartwell, professor at the University of Pennsylvania. In November Western Electric was selected to build a demonstration system, and the first six element array was installed on the island of Eleuthera, Bahamas . Meanwhile Project Jezebel at Bell Labs and Project Michael at Columbia University focused on studying long range acoustics in the ocean.

By 1952 such progress had been made that top secret plans were made to start deployment of six arrays in the North Atlantic basin, and the name SOSUS was first used. The number was increased to nine later in the year, and Royal Navy and USN ships started laying the cabling under the cover of Project Caesar. In 1953 Jezebel's research had developed an additional high-frequency system for direct plotting of ships passing over the stations, intended to be installed in narrows and straights. By 1954 another ten stations were added, including seven in the Pacific, one of which was in Hawaii. A large number of processing stations, known as NAVFACs were also constructed over the next three years.

In 1961 SOSUS tracked the USS George Washington from the United States all the way to the United Kingdom. The next year it tracked the first Soviet diesel submarine to be detected using the system. Later that year the SOSUS test system in the Bahamas was able to track a Soviet Foxtrot class submarine during the Cuban Missile Crisis. SOSUS underwent a number of upgrades over the years, as the quality of the opposing submarines increased.

With the ending of the Cold War in the 1980s, the need for SOSUS disappeared. Most of the system in the Atlantic is now turned off (though still in-place and capable of functioning), although some of the chains are being used to track the vocalizations of whales in various study projects. The system was officially declassified in 1991, although by that time it was long an open secret.

SOSUS systems consisted of bottom mounted hydrophone arrays connected by underwater cables to facilities on shore. The individual arrays are installed primarily on continental slopes and seamounts at locations optimized for undistorted long range acoustic propagation. The combination of location within the ocean and the sensitivity of arrays allows the system to detect acoustic power of less than a watt at ranges of several hundred kilometers.