Extremely Low Frequency (or ELF) refers to a band of radio frequencies from 3 Hz to 300 Hz.
ELF is used by the US Navy to communicate with submerged submarines. The extremely high electrical conductivity of seawater shields submarines from most electromagnetic communications. Signals in the ELF frequency range, however, can penetrate much more deeply. The low transmission rate of most ELF communications limits their use as communications channels; generally an ELF signal serves to request that a submarine surface and initiate some other form of contact.
One of the difficulties posed when broadcasting in the ELF frequency range is antenna size. In order to transmit internationally using ELF frequencies, an extremely large antenna is required. The US maintains two sites, in Wisconsin and Michigan. Both sites use long power lines as antennae, in multiple strands ranging from 14 to 28 miles long. Considerable amounts of power are generated and emitted by ELF, and there have been some concerns over the possible ecological impact of such signals.