ELF, SLF, ULF and VLF
[Dec 16, 2005; Nerd]
Those are the acronyms assigned by the ITU to the electromagnetic frequency spectrum from 3Hz to 30kHz. There is a lot more going on on those frequencies than most people would expect.
There is quite a few stations between 10kHz and 30kHz. Signals get more interesting with lower frequency. Russia and US are communicating with their submarines at 82Hz (ZEVS) and 76Hz (Seafarer). Finally the earth itself is broadcasting at 7.8Hz (Schumann Resonance) and its multiples.
What makes those frequencies so interesting is the fact that modern computers make them easy to receive for everyone. All that is needed is an antenna and a computer with sound card. All the decoding is done in software using FFT.
Radio Waves below 22kHz - good resource for antennas
ZEVS, The Russian 82Hz ELF Transmitter
ELF Transmitter Site Clam Lake, Wisconsin (Seafarer) (PDF)
VLF transmitter DHO38 (German Navy, Rhauderfehn)
Baudline - nice FFT "waterfall" software for Linux