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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.
