Technology developed during World War II when it was discovered that there was a “channel” in the deep ocean within which the acoustic energy from a small explosive charge (deployed in the water by a downed aviator, for example) could transmit over long distances. An array of hydrophones could be used to range on and roughly locate the source of the charge thereby allowing rescue of downed pilots far out to sea. This “channeling” of sound occurs because there is a minimum in the vertical sound speed profile in the ocean caused by changes in the density of the water column. The density is affected by water temperature, pressure (depth), and salinity. Changes in the speed of sound in the water are largely due to changes in temperature and pressure, with salinity offering only a minor effect. See also RAFOS technology.