Underwater, sound travels at a predictable, but varying speed according to depth. Between the surface and around 1km below the surface, it decreases to a minimum. Below this area, called the SOFAR channel, the speed of sound increases again.
The initial decrease is due to the drop in temperature, but the increase is due to the increased pressure the deeper down you go.
To think about this physically, I clapped my hands very loudly and startled myself back into real time.
Final exams week is a killer.