Sonar is a submarine-detecting device that uses sound waves to detect objects underwater. It was first developed by the British during World War I and has since been used by militaries around the world to detect submarines, ships, and other objects in the water. Sonar works by sending out sound waves and then listening for the echoes that are reflected back from objects in the water. By analyzing the time it takes for the sound waves to travel and return, the distance and direction of the object can be determined.