If I understand you correctly, you're concerned about detectable electromagnetic emissions. My reading is that the peak magnetic field is 20 Tesla, not necessa

If I understand you correctly, you're concerned about detectable electromagnetic emissions. My reading is that the peak magnetic field is 20 Tesla, not necessarily that the amplitude of the variations is 20 Tesla. Also, the reason they need to us ELF radio to communicate with submerged submarines is that seawater rapidly attenuates most radio frequencies. Alternatively, if they're varying the magnetic field slowly enough... at the bottom of the ELF band, the wavelength (in a vacuum) is about 8 times the diameter of the Earth, so I'm guessing in seawater that's about 4 times the diameter of the Earth. Localizing such an EM source with enough precision (say, within the destructive radius of a 300 kt nuclear depth charge) to be tactically or strategically useful might prove very difficult if a quarter wavelength is the size of the Earth.