Hi,

>You might also check out "cave radio" and spelunkers (Sp.?) pages.  I
>have read articles in the past about these frequencies.  One thing that
>comes quickly to mind is direct loop to loop transmission.

I think that I was the first to send PSK31 at 1Khz through the Earth.....

My interest was in experimenting with Ohmic "Earth Current" communications
as distinct from Magnetic induction or ElectroMagnet coupling.

I found it very easy  to achieve a few Km range using an audio amp TX and my
laptop soundcard mic input as the RX.

One advantage of the "Earth Dipole" (as I call it) is that it is mainly
resistive and makes a very simple antenna at these low frequencies. A laptop
PC and an earth dipole can make a surprisingly sensitive RX at audio
frequencies. I can easily "see" the CW from VTX3 in India from here in South
Eastern Australia.

John G0AKN has since achieved up to 10 km or so using QRSSS CW as well as
PSK31.

I made a few notes at the time on
               http://www.users.bigpond.com/gzimmer/index.html

Regards .............. Zim .....................VK3GJZ