A narrow channel in which sound waves can be effectively trapped. A region of minimum sound speed is created where the bottom of the thermocline meets the top of the deep isothermic layer. The velocity of sound slows as water temperatures decrease approaching the thermocline from above. The temperature is relatively constant below the thermocline, but increasing pressure causes the speed of sound to increase downwards. This causes obliquely traveling horizontal sound waves to vertically bend back and forth within the sound channel and travel great distances with relatively minor energy loss. This is also known as the SOFAR channel.