ilovefuzz.com

this is certainly the type of weird crap thats going to get me into trouble, errrr.... i mean modular

from the manual:
http://1010.co.uk/org/ERDERDmanual.pdf

and play with the amplification knob.
The earth container will pick up stray electromagnetic signals, particularly if
your hand is placed close to the earth.
Patching in an external earth pile will switch out the local earth container. You
can plug two minijacks between an earth pile or forest site and the earth-labelled
uppermost jack sockets of the module. Any conductive material will also work
with differing properties.

I sort of feel bad for offering a plausible explanation to the magic trick here, but there's LOADS of room for secret sauce so I'm sure I'm not giving away the farm..

The earth is simply going to act like a resistor; actual value is going to hinge on compaction, contaminant/mineral ratio, and water content. I'm sure they are feeding a pretty hot signal via one wire and pulling the other wire with a competent gain section. No need for buried circuitry, you're simply looking at an unusual part.

I've been harvesting the contaminant laden activated charcoal from depleted water filters for a few years now for a conceptually similar application (ie: a resistor) but in a completely different configuration. Depending on DC drop over the material it can get nicely interactive. The next step they should pursue is a CV referenced piston that will alter compression on the material, as that is where the fun kicks in.