It's a little pathway in the ocean where sound can travel a very long way in a very quiet, fairly isolated layer.
Submarines avoid it.
Submarine detection in the sound channel is a big suspenseful part of it. Great film, too. Sound travels faster in warmer water, slower in cooler water. By measuring the time between when a sound is made and when it's heard, scientists can determine the average water temperature along the sound's path. Well, I think it's fascinating.