Communication problems are not new. Research in this area goes back as far as 1922 when the US Bureau of Mines performed experiments to detect radio signals from inside their experimental mine in Bruceton, Pennsylvania. Many improvements to mine communications have been made over the years but for any number of reasons the underground coal mining industry has been behind in implementing advances. Present communications systems for underground mines can be hard-wired or wireless. Both types of systems can fail when faced with fires, roof falls, explosions, and power or battery failure. Currently installed wireless communications systems usually employ a special antenna cable called a "leaky feeder". Fiber optic cables are also used in some applications to form a "backbone" for wireless transceivers. Through-the-earth (TTE) and wireless radio systems are less common. Except for TTE systems, most wireless systems require some wire-bound components, which are susceptible to failure during disasters as cable breakage interrupts communications. A particular requirement for electronic communications systems in underground coal mines is that they need to be designed so they cannot create sparks that might ignite a gassy atmosphere. "Permissible" designs usually involve protection standards such as intrinsic safety or explosion-proof features and may require additional protective features determined by the approving authorities. Permissible design typically limits the output power of wireless devices and makes transmission from the mine to the surface more challenging.