VLF frequencies are from 3hz to 30hz. Which includes the "Schumann resonances" aka the Earth's resonant frequency. It's believed Nikola Tesla used these frequencies to produce unlimited wireless electricity via the ionosphere and a "ground" wire. As far as I know, VLF typically isn't used for communications because there's too much "noise" from the Earth, I found a scholarly article proposing ELF for submarine, but they said 45+hz which would be outside the range of VLF/Schumann.

A potential "conspiracy theory" would be the US Gov stole most of Tesla's belongings from his NY apartment when he died. Some were returned and put in a museum in Serbia, but if anything pertaining to Wardenclyffe or his particle beam weapon were there, I can assure you they kept it. The gov did release some of the documents they took under the disclosures some years back, but most were heavily redacted and pretty much useless. There's been many conspiracies surrounding HAARP in that it was built to replicate the technology.. Obviously, I can't confirm any of this. Although, if I were to build upon the idea, I would say that a potential need for a secret unlimited wireless electricity would be to power hundreds of thousands of remote devices like androids or drones. A simple capacitor circuit and resonant receiver could power literally anything.

https://en.wikipedia.org/wiki/Schumann_resonances https://en.wikipedia.org/wiki/Wardenclyffe_Tower https://en.wikipedia.org/wiki/High-frequency_Active_Auroral_Research_Program