SOSUS

The "Secret Weapon" Of Undersea Surveillance

This is why I never believed that MH-370 went down in the ocean somewhere and no one knew where. If it went down in the ocean I'm sure our Navy could pinpoint it within a few hundred feet.

Twenty odd years ago I remember reading that the Navy had accoustic microphones scattered throughout the world's oceans and we could track virtually any vessel and name it just by the sound of it's screws in the water. It was said that from microphones on the east coast of the USA the Navy could name every vessel entering or exiting the Gibraltar Strait between Spain and Morocco.

Whenever the military admits to having some kind of new tech you have to figure that tech is decades old by the time the public learns of it.