SOSUS
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.
Read more about Sosus.