What is echolocation?
Dolphins and other toothed whales use a sense called echolocation to navigate and hunt underwater, in addition to having extremely good eyesight (except the river dolphins who are bordering on being blind). They emit clicks and use a part of their body called the melon to focus these sound waves on objects around them. Using special cavities in their jaws, they then detect and interpret the echoes that bounce back off. This allows them to build up a picture of their surroundings and can help in locating prey, for instance when it might be hiding under the sand.
Sperm whales also use echolocation to find their way around the dark depths of the ocean and to help with hunting for squid. Though only very brief, the clicks they make are the loudest sound in the animal kingdom.