The speed of sound in water depends on both temperature and pressure. You should get a sound speed profile that looks like Figure 4 . Since the speed of sound varies with pressure, the speed of sound increases as pressure increases with increasing depth. Thus there is a mid-water sound speed minimum, visible in Figure 4. So, now we've found a region where the speed of sound is at a minimum.
Sound waves bend towards a region of minimum sound velocity due to refraction .
Thus a sound channel forms where the speed of sound is at a minimum in the ocean. As we learned above, the speed of sound in water depends heavily on temperature. So how fast the sound arrives at the other side would depend on the temperature of the ocean basin. In this way the scientists could measure the temperature of the whole ocean. Humpback whales migrate over thousands of kilometers.