..............coal miners?

The technical requirements of ELF would seem too burdensome for use in the tight, hard-to-maneuver spaces of the mines. The technology requires a huge antenna, for starters, and has thus been found to have very limited usefulness in analagous situations on submarine craft. In addition, at best the ELF technology can communicate at a very slow rate, only a few characters per minute. In an urgent, life-threatening situation, this technology would likely provide more frustration than assistance.