GOOGLE PATENTS: Loran-based underground geolocation, navigation and communication system
Abstract
A system is provided for underground mapping, location determination and communications utilizing existing LORAN transmitters and a subterranean H-field antenna coupled to a conventional LORAN receiver. The result is an underground LORAN grid from which mapping and location can be ascertained as well as terrestrial-to-subterranean communications using the LORAN bit streams. Subterranean-to-terrestrial communication is established by a low-frequency handheld transmitter using repeat processing to transmit digital data from the subterranean location to the surface of the earth using modulated H-field waves.
Aplanetruth.info 
A old time submariner told me that subs even when underwater back in the 70’s and 80’s could communicate with an underground antenna system in the US. We had a communications company together back then and he explained alot of the tech stuff to me. Apparently this US based underground antenna system started in Michigan and went thru a few states if I recall properly. He was the communications man on subs. I’ll have to look him up again.
LikeLike
Appreciate your website by the way. You might want to look at some of my stuff on my You Tube channel The Flat Earth Survival Guide— https://www.youtube.com/channel/UC7GvPR_azJzke1Ic3zj4E5w/videos.
LikeLike