Radio through the ground was used by radio hams during WWII, because ham radio though the air was prohibited during the war. People would drive two stakes into the ground about 50 feet apart and talk cross-town.
I looked into this "earth mode" radio recently and even tried it out in my backyard without too much success. My question is, does the FCC have jurisdiction over this? Seems like it's not really radio so much as making a circuit using the earth as the conductor.