SOSUS
The "Secret Weapon" Of Undersea Surveillance
https://www.public.navy.mil/subfor/underseawarfaremagazine/Issues/Archives/issue_25/sosus.htm
the Navy’s pioneering Sound Surveillance System – SOSUS – became a key, long-range early-warning asset for protecting the United States against the threat of Soviet ballistic missile submarines and in providing vital cueing information for tactical, deep-ocean, anti-submarine warfare.
As part of the upsurge of ocean-acoustic research that accompanied the coming of World War II, Ewing and his colleagues performed a variety of at-sea experiments that further confirmed sound propagation in the deep sound channel, while also discovering the phenomenon of the near-surface convergence zone.1 On the basis of these experiments, Ewing proposed in 1943 that the Navy develop a system for communicating over long ranges by detonating time-coded explosive charges in the sound channel itself.