Cave-Link: Underground text communication system
Extremely / Very Low frequency radio is fascinating and is surprisingly not used that often. Radio propagation characteristics include being able to pass through water and ground, and continually bounce across the ionosphere / circle the globe. The navy experimented with this for some time to talk to submarines but seems to have ended the experiment.Additionally I learned that lightning strikes generate a high burst of low frequency radio and commercial lightning detectors (which I use while hiking) actually is just a radio receiver on a harmonic of the lightning signal.