In the 1890s, inventor and radio and electricity pioneer Nicola Tesla experimented with using a magnetic wave radio transmitter -- that is, an electromagnet whose strength is modulated to alter the frequency of signals -- as an alternative to Marconi's wireless telegraph. The magnetic wave transmitter turned out to be impractical for regular above-ground communication, because it had a short range and produced a noisier signal. But electrical engineers revived Tesla's concept for another use, for which it turns out to be well-suited -- blasting signals through solid rock and earth to communicate with trapped miners.
In 2006, after rescuers struggled for two days to locate trapped miners in Sago, W.V., Lockheed Martin engineers dreamed up a device, the MagneLink Magnetic Communication System, a refrigerator-sized magnetic generator that can send a signal just like a cell tower would, except it can transmit it a third of a mile underground [source: Berardelli]. In June 2010, Lockheed conducted a successful demonstration of the device at the National Institute of Occupational Safety and Health's test mine in Bruceton, Pa. Subjects were able to send and receive both voice and text messages from 1,550 feet (472.4 meters) underground [source: PR Newswire].