You made me go and look
. The wiki is very informative. Here is the relevant part:====
Extremely low frequency
Electromagnetic waves in the ELF and SLF frequency ranges (3–300 Hz) can penetrate seawater to depths of hundreds of meters, allowing communication with submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system, Seafarer, which was a variant of a larger system proposed under the codename Project Sanguine,[1] operated at 76 hertz,[2] the Soviet/Russian system (called ZEVS) at 82 hertz.[3] The latter corresponds to a wavelength of 3,658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be feasibly constructed.
Instead, one has to find an area with very low ground conductivity (a requirement opposite to usual radio transmitter sites), bury two huge electrodes in the ground at different sites, and then feed lines to them from a station in the middle, in the form of wires on poles. Although other separations are possible, the distance used by the ZEVS transmitter located near Murmansk is 60 kilometers. As the ground conductivity is poor, the current between the electrodes will penetrate deep into the Earth, essentially using a large part of the globe as an antenna. The antenna length in Republic, Michigan, was approximately 52 kilometers (32 mi). The antenna is very inefficient. To drive it, a dedicated power plant seems to be required, although the power emitted as radiation is only a few watts. Its transmission can be received virtually anywhere. A station in Antarctica at 78° S 167° W detected transmission when the Soviet Navy put their ZEVS antenna into operation.[3]
Due to the technical difficulty of building an ELF transmitter, the U.S., Russia and India are the only nations known to have constructed ELF communication facilities. Until it was dismantled in late September 2004, the American Seafarer, later called Project ELF system (76 Hz), consisted of two antennas, located at Clam Lake, Wisconsin (since 1977), and at Republic, Michigan, in the Upper Peninsula (since 1980). The Russian antenna (ZEVS, 82 Hz) is installed at the Kola Peninsula near Murmansk. It was noticed in the West in the early 1990s. The Indian Navy has an operational ELF communication facility at the INS Kattabomman naval base to communicate with its Arihant class and Akula class submarines.[4][5] The British Royal Navy once considered building their own transmitter at Glengarry Forest, Scotland, but the project was cancelled.
ELF transmissions
The coding used for US military ELF transmissions employed a Reed-Solomon error correction code using 64 symbols, each represented by a very long pseudo-random sequence. The entire transmission was then encrypted. The advantages of such a technique are that by correlating multiple transmissions, a message could be completed even with very low signal-to-noise ratios, and because only a very few pseudo-random sequences represented actual message characters, there was a very high probability that if a message was successfully received, it was a valid message (anti-spoofing).
The communication link is one-way. No submarine could have its own ELF transmitter on board, due to the sheer size of such a device. Attempts to design a transmitter which can be immersed in the sea or flown on an aircraft were soon abandoned.
Due to the limited bandwidth, information can only be transmitted very slowly, on the order of a few characters per minute (see Shannon's coding theorem). Thus it is reasonable to assume that the actual messages were mostly generic instructions or requests to establish a different form of two-way communication with the relevant authority.
