> 2) sound can be listened to in the oceans over far longer distances than a useful return signal can be gleaned by the emitter, and tends to transmit unevenly

> 2) sound can be listened to in the oceans over far longer distances than a useful return signal can be gleaned by the emitter, and tends to transmit unevenly due to factors that are difficult to impossible to predict or control like thermoclines, currents, etc. [https://oceanservice.noaa.gov/facts/sound.html https://oceanservice.noaa.gov/facts/sound.html] This sounds exactly like the kind of thing a top secret military program would know how to do with a few petaflops of calculating power, a network of linked microphones, and a complete survey of the area. At least to ranges that would be very surprising to someone outside of that program. But yes, anything some rando on hn thinks of in five minutes has been thought of and dismissed or thought of in detail and had problems said rando didn't think of worked through.