Very interesting question! LF and VLF frequencies are indeed useful for through-the-earth communication under certain circumstances in mines, caves, and even tunnels.There are solutions available for combinations of surface to underground comms and/or location services, through up to several hundred feet of earth or rock. Some of these are also useful for underground to underground signaling. Some work with large loop antennas at one end of the path and loops of various sizes at the other end. Others involve injection of currents through electrodes buried in the earth.
Within tunnels where the route remains consistent over time, a single run of insulated wire running along the length of the system can act as a transmit and receive antenna that picks up a bit of signal from a nearby transmitter unit and guides it as a traveling wave to be detected by receivers near the wire, without direct connection. Such a system may even work at medium or shortwave frequencies. In principle, one could even allow (non-energized!!!) subway rails to be the conductor of such waves, but apart from the risk of contact with energized rails, the sad fact is that such rails will also conduct RF noise from some of the greatest noise generators in the world: the subway car motors themselves.
Many of the old-time LWCA members have also been members of the British Cave Radio Electronics Group ( bcra.org.uk/creg/index.html ) at various times. Their journal provides relatively accessible online articles on a number of different approaches.
You've undoubtedly already seen the Wikipedia article on this topic, but I'll go ahead and present the link to it here for newcomers who may want an introduction:
en.wikipedia.org/wiki/Through-the-earth_mine_communications
John