In early January 2006, a methane explosion tore through a coal mine in Sago, West Virginia, trapping 13 miners nearly 100 meters underground. Cut off from communicating with the miners, authorities could not determine where they were—or even if they were still alive. By the time rescuers reached the miners 2 days later, all but one had died.
After the incident, Gary Smith, a retired engineer, sent a letter to his ex-manager at the Lockheed Martin Corp. in Syracuse, New York. Smith, who grew up in a West Virginia mining family, asked his former colleagues if anyone knew of a technology that could provide reliable communications during such disasters. After reading reports of the Sago incident and discussing similar emergencies with federal mine safety officials, the Lockheed Martin engineers updated a very old one.
The team focused on a concept developed over a century ago by Nikola Tesla. The noted pioneer in electricity and radio had shown that a magnetic wave generator could be used for wireless communications.
Basically, the generator works like an electromagnet. Powered by standard alternating current or battery, it runs electricity through a wire that is coiled around a metal cylinder, creating a harmless, low-energy magnetic field that extends for hundreds of meters. Just like radio, the field can carry an audio signal by modulating (raising or lowering) its strength instant by instant. But unlike radio, cell phones, and satellite phones—whose electromagnetic waves can’t pass very far through rock, clay, or other materials that conduct electricity—a magnetically generated signal penetrates the ground easily. On the other end, a coiled antenna wire about 100 meters long receives the signal, and an amplifier converts it into sound.
Read more here: ScienceNow