I noticed the US Army has
announced its intent to award Canada's Ultra Electronics Marine Systems a contract for "Rock Phones". These magneto-inductive communication systems can work through rock and earth that radio cannot penetrate.
Been done before ages ago, admittedly in the usual Heath-Robinson way - Once again Johnny foreigner makes the dosh out of a british invention!
SOME HISTORY
Radio-communication through solid or liquid substances is not that simple! The higher the frequency, the better the surrounding matter will absorb the radio waves. This is why Walkie-Talkies won't work, unless the transmitter and receiver can "see" each other.
That's why cave-radios use antennas that are made of cable, wound in a small loop.
In the 70ties, some British cavers started experimenting with VLF (Very Low Frequency) transmitters to try to establish radio-communications between the surface and a cave. After some years of trying and inventing the right antennas, they succeeded in their plans. These first cave-radios were pretty big and used a lot of (battery)power, so cavers were dragging around considerable weights! But things soon got smaller and lighter. Two popular designs were the Ogophone and the Molephone, capable of reaching depths around 150-180 m.
The Molephone was commercialized and can still be bought. The design is secret. The design of the Ogophone however was published, and amateurs can still rebuild this type of radio. The antenna is a loop with a diameter of 1 metre, made out of 64 turns of computer- flat cable. The radio is build into an ammunition box, since British cavers like to drag these things around. The battery is put in a separate box.
CURRENT SITUATION
Currently, cave radio's are commonly used in Great-Britain, especially by the Cave Rescue Organisation. No more dragging telephone wires around: the Rescue team leaves with two ammo-boxes, plugs in the microphone and antenna, and they have a vocal communication with the surface-team. If necessary, the receiver (surface) is connected to a normal telephone or G.S.M. so the underground Rescue-team can speak to a doctor or hospital!
In Great-Britain there is a user group, called the CREG (Cave Radio & Electronics Group). They are a division of the BCRA (British Cave Research Association). These guys like to experiment with cave radios. In fact, the predecessor of the cave radio, a loop-wound antenna that gave a 3kHz "beep-beep" signal, was born here. It was used to for radio-positioning (positioning an underground point at the surface).