CNN CNN Newsroom April 12, 2014 14:25:00
Salinity, temperature, and if sound is coming up here, and it hits this layer. because this is slower, it could essentially bend that sound and make it go to a different angle than it was a moment ago, and then it may come out where you don't expect it at all. also, in some cases, it may come up, hit the layer and start ping-ponging between the top and the bottom of it. and by some theories, particularly with lower sounds, it may go many, many, many miles before it comes out. this may explain why a pinger that shouldn't be heard more than three miles away is being located in a range of some 17...
Source: archive.org