Interesting article. I think it's overwhelmingly likely that these surveillance robots are obtaining detailed information about water salinity and temperature,

Interesting article. I think it's overwhelmingly likely that these surveillance robots are obtaining detailed information about water salinity and temperature, possibly as well as acoustic noise. They are also oddly beautiful. Salinity and temperature are of critical importance in computing the location of natural waveguides that occur when a reversal of the sound speed gradient occurs at depth in the thermocline, corresponding to the minimum sound speed at that vertical position for a very large horizontal distance. In temperate waters, this can be quite close to the surface -- i.e. within the comfortable range of military submarines -- leading to the so-called Deep Sound Channels (https://en.wikipedia.org/wiki/SOFAR_channel https://en.wikipedia.org/wiki/SOFAR_channel) where any sound can be readily detected.