Deep sound channel

The deep sound channel or DSC, was a region of the ocean, about eight hundred meters deep here, where temperature, pressure, and salinity conspired to produce the lowest speed of sound. It was used as a medium of communication for marine creatures such as the Selkies and the squales.

Mechanics
Since any wave passing between two media was refracted toward the one where its speed was lower, the DSC tended to confine sound waves inside it as solitons, much as light was trapped within the opti-cable inside the USS Titan’s consoles and computers. Since the waves propagated in only two dimensions instead of three, it took their energy longer to disperse, so sounds emitted in the DSC could travel thousands of kilometers if loud enough.