Based on the author's PhD thesis, this book is a theoretical study of subterranean radio communication, with the focus on methods that depend primarily on the penetration of electromagnetic fields through the ground. Through-the-earth communication using e-m fields - specifically magnetic induction - plays a role in search and rescue systems used in the mining industry. It is also used for borehole telemetry, pipeline location and by cavers and pot-holers. Chapters describe propagation, antennas, and the design of transmitters and receivers. A figure of merit - the specific aperture - is introduced as an aid to antenna design. A crucial aspect of all systems is the signal to noise ratio, for which the strategy of noise-matching is introduced. The design of a wide-band low-frequency sounder is described, for channel evaluation using a non-ideal binary sequence. A method of calculating the inverse of a generalised sequence is described, for which cross-correlation results in a system identification signal.