VLF is an acronym for very low frequency. VLF covers 3 kHz to 30 kHz in the radio frequency range. Most of the radio transmissions in the VLF range are used by the Navy for surface and subsurface communications and navigation.
The signals are transmitted from stations all over the world. They operate at high power and require very large antennas. Many of the signals are encrypted and rarely does a station ID. To receive these signals one would have to modify a conventional radio or build your own. There are expensive commercial receivers available that pick up the VLF range and lower.
VLF Radio Stations:
Very Low Frequency (VLF), (www.fas.org/nuke/guide/usa/c3i/vlf.htm)
Very Low Frequency Radio Stations (moondog.astro.louisville.edu/flares/stations.html)