VLF would also go through rock and water? Could be useful for spelunking (and submarine comms). Depends on if they can transmit without a huge antenna as well.
That would be reinventing James Harris Rogers WW-I system.
T.E.Bearden claimed that this system has been rediscovered and "lost" several times.
that's an interesting footnote...
Check out the list of Curl-Free Magnetic Vector Potential patents by Raymond C. Gelinas assigned to Honeywell.