About very expensive machines that go "PING!".
Underwater Sound Propagation.
As you can see from the diagram, there is an extensive "shadow zone",the gap between where sound is deflected into the "surface duct", or into the depths. A submarine in this shadow zone could not easily detect a ship at (0,0) making a noise, as the sound is bent away from it. Conversely, that ship's active sonar - which goes PING!- couldn't find the sub, as the sound is deflected away from the target too. Not enough energy reaches it.
The article is a good basic guide to the principles, including such phenomena as the Deep Sound Channel, Convergence Zones, and Bottom-bounce techniques.
It's simplified though. Because the bathytherograph may have several discontinuities, lading to several "thermal layers". Salinity as well as temperature can vary, and there may be vertical layers too, meaning the sound travels in a helical pattern!
Finding subs under those anomalous conditions is close to impossible, even with the best equipment, as there's no sound to detect. Conversely, detection may be possible using a variety of techniques simultaneously, or you can get "multi-path" propagation, where sound of different frequencies takes different paths, so the signal is "smeared".
It's a specialised area, but one I've had something to do with in the past, and one I find quite fascinating.
Even if no-one else does.