Through-the-Earth (TTE) signalling is a type of radio signalling used in mines and caves that uses low-frequency waves to penetrate dirt and rock, which are opaque to higher-frequency conventional radio signals.
In mining, these lower-frequency signals can be relayed underground through various antennas, repeater or mesh configurations, but communication is restricted to line of sight to these antenna and repeaters systems.
Through-the-Earth transmission can overcome these restrictions by using ultra-low frequency (300-3000 Hz) signals, which can travel through several hundred feet of rock strata. The antenna cable can be located on the surface only at a mine site, and provide signal coverage to the mine. The antenna may be placed in a "loop" formation around the perimeter of the mine site (or wherever coverage is needed) for systems using magnetic fields to carry signals. Systems that use electric fields as the signal carrier are not subject to this limitation. Transmissions propagate through rock strata which is used as the medium to carry the ultra-low-frequency signals. This is important in mining applications, particularly after any significant incident, such as fire or explosion, which would destroy much of the fixed communication infrastructure underground.
If the terrain makes a loop surface antenna impractical to install, then the antenna can be installed underground or a non-magnetic field type carrier may be used. But because the signal travels through rock, the antenna does not need to run into all parts of the mine to achieve mine wide signal coverage, thus minimizing the risk of damage during an incident.
Portable magnetic-loop cave radios have been used by cavers for two-way communication and cave surveying since the 1960s. In a typical setup the transmitting loop, consisting of many turns of copper wire, is oriented horizontally within the cave using a spirit level, and driven at a few kHz. Though such a small antenna is a very poor radiator of propagating radio waves at this low frequency, its local AC magnetic field is strong enough to be detected by a similar receiving antenna up to a few hundred meters away. The received signal's strength and its dependence on orientation of the receiving coil yields approximate distance and directional information. Systems like Molefone and HeyPhone operate as single-sideband (USB) 87 kHz radio with earthed electrodes of 25-100 m length. Design for HeyPhone system is openly available.
There are several systems that have been recently developed. One system is known as the PED System, where PED is an acronym for personal emergency device. Initially developed after a mining disaster in Australia at Moura No. 4 Coal Mine in 1986, and further developed after the Moura No. 2 Coal Mine explosion in 1994  where the need for a communication system to survive major incidents underground was identified in the inquiries into the disasters.
PED is a one-way text paging device, with wide use in Australia, as well as installations in the United States, China, Canada, Mongolia, Chile, Tanzania, and Sweden. Australian company Mine Site Technologies began the development of PED in 1987, and it became commercially available and Mine Safety & Health Administration (MSHA) approved in 1991. The best documented use of PED during a mine emergency is from the Willow Creek Mine Fire in 1998 in Utah, where it was able to quickly alert miners underground of the need to evacuate before toxic fumes from the fire filled the mine. Reports of this use can be seen on the MSHA website.
Emerging technologies have recently been developed such as the Rescue Dog Emergency Through the Earth Communication System  developed by E-Spectrum Technologies. The Rescue Dog is a two-way extended-range portable through-the-Earth solution that was developed in the US in cooperation with The National Institute for Occupational Safety and Health (or NIOSH)  which does not rely on large loop surface antennas for signal transmission. New non-portable systems have also been developed by companies such as Lockheed Martin for use in emergency chambers to provide post-accident, two-way, emergency voice and text communications independent of surface or in-mine infrastructure.
A new wireless "Miner Lifeline" telecommunication technology is being tested in 2012 at the West Virginia Robinson Run mine (recent production 6,000,000 short tons (5,400,000 t) per year of coal using 600 miners). The system supports voice, text, or SOS sent on a "bubble" of magnetic waves, and "can move more than 1,500 feet (460 m) up or down and 2,000 feet (610 m) laterally, arriving in less than a minute."