• Also, a phenomenon in the ocean, known as the thermocline, which acts as a horizontal acoustic mirror below the surface makes it difficult for sound to propagate from the deep ocean up through this layer.

Thank you for posting this. I was utterly confused on a couple of things, and this explains a lot. (Not to mention why the "pings" allegedly heard from the people from China with a pringles can on a stick have been thoroughly dismissed)..