Bit of an understatement. ELF (Extremely low frequency) requires finding a section of earth's crust with very low conductivity that can be repurposed as an ant

Bit of an understatement. ELF (Extremely low frequency) requires finding a section of earth's crust with very low conductivity that can be repurposed as an antenna. These antennas are very large, the US had one that was 45km long, the Soviet antenna is 60km long. The resulting signal has a wavelength of 3500-4500 KM, about a quarter of the earth's diameter. Because of this huge wavelength, the bandwidth is extremely low. The US system took 15 minutes to transmit a single 3 letter message. The system was only really useful for transmitting pre-agreed signals (which were usually along the lines of "surface and contact us for the actual orders"). But they also acted as a dead-mans switch, they continually transmitted an idle signal. If they stopped transmitting (which would still take several minutes to notice) subs would establish communication to check if a nuclear war had started.