SOSUS, an acronym for Sound Surveillance System, is a chain of underwater listening posts across the northern Atlantic Ocean near Greenland, Iceland and the United Kingdom — the GIUK gap. It was originally operated by the United States Navy for tracking Soviet submarines, which had to pass through the gap to attack targets further west. Other locations in the Atlantic and Pacific Ocean also had SOSUS stations. It was later supplemented by mobile assets such as the Surveillance Towed Array Sensor System (SURTASS), and became part of the Integrated Undersea Surveillance System (IUSS). Many other listening posts are still in operation around the world.
SOSUS development was started by the Committee for Undersea Warfare in 1949. This panel was formed by the Navy to research anti-submarine warfare. They allocated $10 million annually to develop these systems. The primary threat was diesel submarines; the Soviets were building a large fleet. The group decided to monitor the SOFAR channel to detect low-frequency sounds, which would be effective over hundreds of miles. Listening sites had multiple hydrophones and a processing facility. This allowed estimating submarine positions by triangulation.
At MIT in 1950, the committee sponsored Project Hartwell, named for the director of the committee, Dr. G.P. Hartwell, physics professor and president of the University of Pennsylvania. In November, they selected Western Electric to build a demonstration system, and the first six element hydrophone array was installed on the island of Eleuthera in the Bahamas. Meanwhile Project Jezebel at Bell Labs and Project Michael at Columbia University focused on studying long range acoustics in the ocean.
By 1952, enough progress resulted in top secret plans to deploy six arrays in the North Atlantic basin, and the classified name SOSUS was used. The number of arrays was increased to nine later in the year, and Royal Navy and USN ships, including USS Neptune and USS Peregrine, 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 straits, called Project Colossus.
In 1961, SOSUS tracked the USS George Washington (SSBN-598) from the United States to the United Kingdom. The next year SOSUS detected and tracked its first Soviet diesel submarine. Later that year the SOSUS test system in the Bahamas tracked a Soviet Foxtrot class submarine during the Cuban Missile Crisis. SOSUS was upgraded a number of times as submarines became quieter.
SOSUS systems consisted of bottom mounted hydrophone arrays connected by underwater cables to facilities ashore. The individual arrays were 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 allowed the system to detect acoustic power of less than a watt at ranges of several hundred kilometers.
SOSUS monitoring stations were known as Naval Facilities (NAVFAC - not to be confused with the Naval Facilities Engineering Command that has the same acronym.) NAVFACs existed in the west at Adak, Alaska; Pacific Beach, Washington; Coos Bay, Oregon; NAVFAC Centerville Beach near Eel River, California; NAVFAC Point Sur near Monterey, California; San Nicolas Island, California; and Naval Air Station Whidbey Island, Washington (1987). In the east, they were deployed at Tom Nevers Naval Facility Nantucket; Lewes, Delaware; Cape Hatteras; and Naval Facility Punta Borinquen, Puerto Rico.
Other NAVFACs were located in the Pacific at Barber's Point, Hawaii, Midway Island; and Naval Base Guam. Also, they were located in the Atlantic at Keflavik, Iceland; Naval Station Argentia, Newfoundland; CFB Halifax, Nova Scotia; Brawdy, Wales; Antigua; Barbados; Eluthera, Bermuda; Grand Turks; and San Salvador. Data Evaluation Centers were set up at Whidbey Island, Washington and Dam Neck, Virginia in the early 1980’s.
LOFAR (frequency analysis) was carried out on the signals from the arrays and paper outputs ( lofargrams) were produced which were used to help detect and classify contacts. When the USS Thresher (SSN-593) sank in 1963, SOSUS helped determine its location. In 1968 the first detections of Victor and Charlie class Soviet submarines were made, while in 1974 the first Delta class submarine was observed.
In 1985, the Fixed Distributed System (FDS) test array became operational and the first SURTASS patrol began. The name for the overall system became Integrated Undersea Surveillance System (IUSS). In 1991, the system mission was declassified and the next year it began reporting whale detections and SOSUS work stations began replacing paper lofargrams. The Advanced Deployable System became operational as part of IUSS in 1996.
SOSUS was gradually condensed into a smaller number of monitoring stations during the 1970s and 80s. However, the SOSUS arrays themselves were based upon technology that could only be upgraded irregularly. With the ending of the Cold War in the 1990s, the immediate need for SOSUS decreased, and the focus of the US Navy also turned toward a system that was deployable on a theater basis. The SOSUS components are now used for scientific projects, such as tracking the vocalizations of whales and other ocean mammals in various study projects, as a data network for undersea instrumentation packages, and for acoustic thermometry. The SOSUS system was declassified in 1991, although by that time it had long been an open secret.
Commander Undersea Surveillance (CUS), head of the IUSS, was elevated to an echelon IV command 28 February 2007.  CUS at NAS Oceana Dam Neck Annex operates under the operational guidance of COMPACFLT. Naval Ocean Processing Facilities in Oak Harbor, Washington, and Virginia Beach, Virginia, still monitor SOSUS and FDS, and they provide SURTASS connectivity around the world.