What is the definition of earth-mode?

I could not find one, but my understanding is "communication between two points essentially using conduction through the ground". In the modern world it is almost impossible to find anywhere, except remote moors, that is not criss-crossed with buried cables and pipes. My interpretation of earth-mode therefore includes utilising these as a means of helping a signal get from (a) to (b). Hence my term "utilities assisted earth-mode". In mining and military use of earth-mode I think this is also accepted. What I also mean is communicating from (a) to (b) not using a radiated signal or induction as the prime means of propagation.

Why experiment with this form of VLF/ULF communication?

From my tests last year it was clear that local communication "piggy-backing" on these buried utilities seemed to be possible over many kilometres without the need for large and complex antenna systems that are essential to get even a tiny VLF signal actually radiated, as Ossi, Gerhard, Stefan and Chris know only too well.

Questions I'm trying to answer are: