Radio waves in the extremely low frequency (ELF) band of 30 to 300 Hz can penetrate to a depth of hundreds of meters, allowing them to communicate with submarines at their normal operating depth. The lower the frequency, the longer the wavelength of the radio waves, and transmitters require longer antenna structures to generate them. ELF transmitters use huge antennas called ground dipoles consisting of tens to hundreds of kilometers of overhead cables resembling ordinary power transmission lines. The transmission lines are grounded at the ends, and looping currents deep in the Earth form part of the antenna. Because even these huge antennas are much smaller than the ELF wavelengths, they are extremely inefficient; only a tiny fraction of the input power is radiated as ELF waves, with the rest dissipated as heat in antenna resistance. At their full input power of 2.6 MW, both US ELF transmitters working together only generated about 8 watts of ELF radiation. This weak signal was able to reach submarines over half the globe only because of the extremely low attenuation of ELF waves of 1-2 dB per 1000 km. ELF transmitters are most efficient when sited over certain low conductivity underground rock formations, which forces the currents to spread deeper through a larger volume of rock, forming a larger "antenna". The US system was located in Wisconsin and the Upper Peninsula of Michigan over the Laurentian Shield formation, for that reason.
The US Navy ELF Transmitters are located in Clam Lake Wisconsin, and in Republic Michigan located in Michigan's upper peninsula.
To maximize the system's potential they operated simultaneously in tandem to create a more powerful signal. There are established biological effects from acute exposure at high levels (well above 100 µT) that are explained by recognized biophysical mechanisms. External ELF magnetic fields induce electric fields and currents in the body which, at very high field strengths, cause nerve and muscle stimulation and changes in nerve cell excitability in the central nervous system.
But there is a less talked about side effect from ELF communications. Certain conditions combined with particular messages form something strange. Initially dismissed by the Navy operating the system. It has been realised to be true by a few of the Navy researchers. The system can open passages to somewhere else. Other dimensions, worlds, the veil, or whatever you want to call it. Things have come through. Things that cannot be explained. Most fade out with time. But some continue to haunt the wilds. Searching, doing inscrutable things.
This is the mysteries that the characters have fallen into. What do they want? Can they be stopped? Do they need to be? Then there's the government....