Why does the Earth act as a near-perfect conductor for electric current?

[FrankMak]

I have been examining literature related to charge redistribution at the Earth's surface and I found an application that treats the Earth as a near perfect conductor.

http://tdworld.com/ar/power_one_wire_enough/
www.stonepower.se/Images/SWER.pdf

The articles in the above URLs are discussing an active application of the Earth's near zero resistance to an electric current. It is a working application that can't be disputed, but I cannot find reference sources that identify the theory for the near perfect conduction.

Any sources for the Earth conduction theory?

 

[CWatters]

Various authors/papers on SWER systems cite:

Rudenberg, R., "Fundamental Consideration on Ground Currents,"
Electrical Engineering, Vol. 64, January 1945, 1-13.

And typically refer to a figure of 0.05 Ω/km.