ELF: an acronym for Extremely Low Frequency, a band of the electromagnetic spectrum defined by the IEEE as 30 to 300 hertz, although frequencies of up to 3 kilohertz are sometimes referred to as ELF as well.

ELF frequencies are produced naturally in the earth's ionosphere, among other places; and artificially by various sources including power lines. Active communication in these frequencies is very difficult because the antennas used in their creation must be enormous. Nevertheless, the United States Navy once operated its nuclear submarine communication systems in the ELF range, to prevent the attenuation that higher-frequency signals are prone to in deep water.(The program was eventually terminated due to high costs, the end of the Cold War, and wildlife protesters.Thanks to unperson for pointing this out.)

There is inconclusive evidence suggesting that ELF fields produced by overhead power lines may be a cause of cancer, and the effects of ELF on living organisms are a subject of intensive research - some claim the submarine communication systems injured the auditory apparatus of marine mammals. The ELF band is also a favorite topic of discussion among conspiracy theory enthusiasts who believe it can be used for mind control.