Sound bounces off of hard surfaces, or when there is a change in density. Bats, fishermen and submarine captains use this fact as part of echolocation. By timing how long it takes for the echo of a sound to return and knowing how fast sound travels, the location of an object can be calculated. This is a form of scanning.
If the speed of sound in water is 1500 m/s and it takes 0.2 seconds for an echo to return, to what depth should the fishermen cast their nets?
Use: Speed = Distance / Time