Radio waves, high-frequency electromagnetic oscillations, are powerless against the Earth's bulk: they attenuate, being almost completely absorbed within a couple of meters of depth. But a low-frequency magnetic field, governed by the equations of electrodynamics, behaves differently. It penetrates the ground like an invisible bridge thrown across rocks and soil. This effect, predicted and described within classical electrodynamics by James Clerk Maxwell and experimentally discovered by Michael Faraday, underlies underground communication: a low-frequency electromagnetic field barely notices obstacles that become walls for radio waves.

However, such a bridge has a fundamental limitation: the signal attenuates exponentially, and after just a few meters its amplitude drops below the level usable for detection. The engineering solution is to install a relay — a kind of 'support' that receives the weakened signal, amplifies it, and sends it onward. This extends the magnetic bridge tenfold, turning it from a local crossing into a long communication line. The key to efficiency lies in the mathematical framework: calculations using entropy, a measure of uncertainty or information capacity of a channel, show that data transfer rates increase even if the relay is installed with deviations from ideal geometry. In other words, the system proves remarkably robust to imperfect placement.

A striking contrast with radio waves emerges in moist environments: the magnetic field passes through water-saturated soil even more easily than radio waves do through dry soil. This feature paves the way for fast data transmission in subways, mines, and exploration of underground rivers — a kind of 'Wi-Fi through rock,' where the information carrier is not a radio wave but a low-frequency magnetic field modulated with the useful signal.