Radio waves die out in the ground after just a couple of meters. But magnetic fields pierce right through, like an invisible bridge. This bridge is what enables communication in mines and caves. The effect was discovered by Michael Faraday and described by James Clerk Maxwell: low-frequency electromagnetic fields barely notice obstacles.

The problem is that the bridge is short — the signal fades after a few meters. Engineers installed a repeater — a support that catches the faint signal and sends it onward. This extends the bridge by tens of times. The secret is in the math: a calculation using entropy (a measure of uncertainty) shows that the speed increases even if the support is positioned crookedly. Amazingly, through moist soil, magnetic fields travel more easily than radio waves through dry ground.

Now, in subways, mining tunnels, and when exploring underground rivers, it’s possible to set up fast data transmission — like Wi-Fi, but through solid rock.