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Reminds me of a presentation I attended back in the mid 70’s at the Baltimore Grotto (a caving club affiliate of the National Speleological Society). Some ham/cavers built a few portable AM transmitters that broadcast at the low end of the AM band. They got an experimenter’s license from the FCC and did tests to determine how best to use these radios for communication between ground-level and inside a cave. The low frequencies worked just fine. The real drawback was having to string out a long piece of wire for the antenna.
I think they were going to do further trials with loop antennas. I don’t know what the results were, but I suspect a proper loop would be more convenient, although more problematic, due to the nulls.
BTW, the US Navy uses VLF for communicating with subs while under the water. They have huge antenna arrays for this. Of course, the drawback with VLF is the low data rate.