The appropriately-named SOFAR channel is a level of the ocean that allows whale calls to travel hundreds of miles, and lets a recording station in California record an underwater earthquake in Hawaii. This is how it works.
Sound is a wave, and it can bend like a wave. To understand how, imagine speeding down an even road on a pair of roller skates. Your right skate hits a rough patch and slows down, causing you to swing around to the right. If you were going down a rough road, and your right skate hit a smooth patch, it would move faster, and you'd swing around to the left. (Naturally if both your skates hit a rough patch at the same time, you travel more slowly than you did over the smooth section, but you stay on course.) The same thing happens with sound or light or other waves. When a wave hits a material that lets it travel faster or slower than its current medium it turns.
The wave doesn't require a new kind of material to bend. The same material at different pressures or temperatures will force a wave to travel at different speeds. For example, when water is at a lower temperature, sound waves move through it more slowly. But when water is at a higher pressure, water will travel through it more quickly.
This causes a phenomenon in a level of the ocean called the SOFAR layer, or the SOFAR channel. SOFAR channel stands for the SOund Fixing And Range channel. It occurs between 600 and 1200 meters below the surface. Sound waves that stray into its range from above suddenly bend down as the temperature drops and the speed of sound in the water below them decreases. They travel down and down, until they get to a depth at which the water pressure increases. This causes the speed of sound to increase, swinging the sound wave back upwards, only to be driven down again when it nears the top of the SOFAR layer.
This channel allows sound waves to move without dissipating much of their energy. More energy allows the sound wave to travel a longer distance. This SOFAR layer is what allows whale song to travel through the ocean. (Some say that, before noise pollution in the ocean, whales could communicate from one pole to the other.) It also lets scientists listen in on seismic events far away.
[Source: Ocean Explorer]