Sound Surveillance System

Definition
The SOund SUrveillance System (SOSUS) provides deep-water, long-range detection capability. SOSUS enjoyed tremendous success during the Cold War tracking submarines by their faint acoustic signals. SOSUS consists of high-gain, long fixed arrays in the deep ocean basins.

Source

 * FAS Intelligence Resource Program, "https://www.fas.org/irp/program/collect/sosus.htm" (full-text).