The VLF radio theory can be tested this evening, without special materials or technology. VLF radio waves (between 3 kHz and 30 kHz) can penetrate earth to considerable depths, depending on the conductivity and magnetic permeability of the ground. Therefore, if one descends deeply enough underground, VLF signals are fully blocked, and this Hum hypothesis can be tested. There are passing references in a few Hum articles to people going deep into limestone caves, but not a lot of detail is available on those investigations.
The sharp reader might ask: wouldn’t this block sounds as well, so if the Hum is silenced deep underground, how do we know what the source is? It turns out that sounds can travel well through solid rock. This doesn’t perfectly sort out VLF radio from acoustic sources, but it helps.
From a quick scan of the VLF literature, it seems that VLF radio has a maximum earth penetration of no more than about 150 metres (about 500 feet). If a reader familiar with this literature has other information, by all means, pass it along. A few years ago I began discussions with the group that oversees a deactivated copper mine close to here, but I turned my attention elsewhere. It may be time to open those lines of communication again.
There are many places in North America and Europe where deep caves or deactivated mines are found, although access might be problematic (or illegal) in some cases. Also, if one of you does get permission, or the nerve, and get a guided tour to go underground, note that all mains electricity and other equipment need to be shut off. Running water of any kind will also skew the results. And please be careful.