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Very low frequency (VLF) is a radio frequency band in the range of 3 to 30 kHz with wavelengths from 10 to 100 km. It is also the name of the band designated by the International Telecommunication Union (ITU). VLF has a limited bandwidth so it is mostly impractical for audio transmission and is mostly only used for coded signals with a low data rate such as underwater communication.
Very low frequency signals have large wavelengths that allow them to diffract around large obstacles, propagate as ground waves along the earth's curvature and penetrate salt water, which is why they have been used by the military for communicating with submarines. In fact, the most powerful radio station in the world was designed to transmit VLF signals at 24 kHz at a power of 1.8 MW. This station is the US Navy's Naval Radio Station Cutler located in Cutler, Maine, which features an antenna array 1.2 miles in diameter with a central mast used as the radiating element and a star-shaped horizontal wire array acting as a capacitive top load. It is used to communicate with US submarines.
VLF signals are very special in that they use the ionosphere D layer of the atmosphere at an altitude of 60 km and the conductive earth as a horizontal "duct" waveguide, which confines the waves so that they do not escape into space, allowing them to propagate to very large distances around the earth. This makes VLF a very convenient tool for sensing activities in the D region of the atmosphere where it can be used by scientists to detect phenomena like lightning, solar activity and cosmic gamma rays, but propagating VLF signals in this complex media is the most difficult feat to achieve in the field of electromagnetism. VLF is also used for geophysical surveys, which use electromagnetic waves to detect what is under the ground as well as the ground's composition.