Various nations' militaries have used ULF and VLF to communicate with their submarines, but the usual practice is to utilize very large antennae.
DARPA is experimenting with much smaller, portable antennae using the idea of mechanically vibrating electrically or magnetically "charged" objects at ULF/VLF frequencies:
Data rates will naturally be low, but the idea is to permit local communication between divers, in situations involving caves and tunnels, and over vast distances (atmospheric waveguide effect).
I'm a bit skeptical, but it should work presuming their technology goals are met, including much stronger magnets and electrets plus mechanical systems to vibrate them.
Anybody want to bet this ever gets past the laboratory curiosity stage? I guess the only evidence would be the Navy dismantling its VLF antennae.
Mark L. Fergerson