Sofar channel

SOFAR channel stands for "Sonar Fixing and Ranging channel". It's a layer of water deep in the ocean (near Bermuda it's around 1000 m deep) where the speed of sound is at a minimum. Within the SOFAR channel, low frequency waves may travel hundreds, if not thousands, of miles before dissipating. In temperate waters, this minimum sound depth is shallower, and it reaches the surface between about 60 degrees N or 60 degrees S. This phenomenon is probably controlled by the presence of a deep thermocline (or central water thermocline) and a seasonal thermocline (or at least an intermediate thermocline) which form a lower and upper barrier, respectively, to "channel" sound waves. Strange and mysterious low-frequency sounds, attributed to humpbacks and other baleen whales, are a common occurrence here. Scientists think that humpback whales may dive down to this channel and "sing" to communicate with other humpback whales many kilometers away.