This uses VLF RF, but there was another interesting thing I have read about long ago called "earth mode" or "conduction mode" maybe. You put two conductors in

This uses VLF RF, but there was another interesting thing I have read about long ago called "earth mode" or "conduction mode" maybe. You put two conductors in the ground some distance apart (10s or hundreds of feet) and inject the signal into the ground. The signal causes a current in the ground which will cause voltage differentials that can be picked up miles away with a similar receiver. I believe it is also low bandwidth, but I often wonder if you could create a mesh network of these. Also would the FCC actually be able to regulate this since it is not actually a radio wave?