There are competing schools of thought about the source of the Hum. Most of those theories collapse under the weight of some inconvenient facts and very basic questions.
1. Why does the Hum sometimes suddenly disappear, wax and wane, and then reappear? Answer: My hypothesis is that VLF, and possibly ELF, radio signals generate interference patterns and standing EM waves that are interpreted by the body as low frequency sounds. One of the propagation modes (method of travel) of VLF waves is the “skywave”, sometimes called “skip” by amateur radio enthusiasts, in which the radio waves hop back and forth between the ground and an electrically charged layer of the atmosphere called the ionosphere. I also suspect that the interaction between geomagnetic conjugate points is a factor. Any disturbance or change in the ionosphere will change hop distances and signal strength. Such disturbances can include solar storms (linked to the sunspot cycle) and internal processes within the Earth’s core that generate the magnetic field itself. Moreover, powerful VLF transmitters (which as far back as 1965 included portable land-based transmitters) have ceased operation, changed frequencies, or increased in power. Any of these can cause the anomalous behaviour that typifies the Hum.
2. Why did the first widespread and reliable reports of the Hum start in England in the 1970s, and America in the early 1990s? Answer: When we examine the history of the appearance of the world’s most powerful VLF transmitters (land-based and airborne), and ELF transmitters, we find that the two coincide. The precise power of the land-based transmitters is often classified or intentionally obscured, but if they were operating in the 2 MW range as far back as 1960 (which is in the public record), then with little doubt, 50 years of technological advancement have brought not only much greater transmitting power, but greater numbers of such transmitters as well. I estimate that there are at least 50 VLF transmitters in the world that operate in excess of 1 MegaWatt power. The actual number could be greater. Some of those transmitters are continuously airborne (e.g. the US TACAMO system), whose precise power and locations are classified.