I don’t know if any of them ever ended up getting off the ground, but the theory was that you could pick up the step-function shockwave from underwater interactions with a giant array of microphones. There were some test arrays in the Mediterranean IIRC, but cursory searches seem to imply that optical underwater arrays seem to have been pursued instead.

It always seemed a neat idea, that you could instrument absolutely gigantic volumes with.

I guess the sound might be very characteristic because variations in the process of generation might be limited (amount of energy delivered / type of particle created?) and I would expect it to sound a lot like a sharp "ping" if I had to take a guess. Temperature, salinity and currents would affect propagation and dispersion but deep enough in the sea there might be a stable enough environment to be able account for that. A very interesting idea at least!