The Very low frequency reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Very low frequency

Very Low Frequency or VLF refers to Radio Frequencies (RF) in the range of 3 - 30 kHz. Since there is not much space in this band of the spectrum, only the very simplest signals are used, such as for radionavigation. Many natural radio emissions, such as whistlers, can be heard in this band.

Because VLF can penetrate water to a depth of 20 meters, they are used to communicate with submarines.

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