Eric Dollard commented on the very first accidental telluric experiments he performed, before he’d even heard of Tesla.

He was messing with a single Tesla Extra coil (air-core high Q resonant quarter-wave coil), observing all its interesting properties, and got a call from a friend a town over telling him he was ‘on the air’ even though there was no antenna.

That was when he realized the ground was acting as a transmission point.