Any noise coming out of a submarine will reveal its location. Through various detectors (hydrophones) arrayed along the hull, a sub can use triangulation to pin point another submarine, engine noise, cavitation of the propeller (Red October used an impeller interesting concept). Attack subs can also tow a long array of hydrophones to listen to subs much further away.

The US Navy maintains a series of listening devices along the east coast to listen to Soviet subs. It was named SOSUS (SOund SUrveilleance System).