Sonar is a machine that uses sound waves to find other objects under water in the sea. Sonar stands for Sound Navigation and Ranging. The word is now thought by many people as a regular word. Sonar can work in two ways. First, it can be used to find things by sending out sound and listening for echoes. This is usually called active sonar. Second, it can search for things by listening for sound made by the object it is trying to find. This is called passive sonar.
The most important use of sonar has been in wars at sea (naval warfare). Ships can use sonar to find other ships or submarines under the water. One problem with sonar is that enemy ships and submarines can detect when the sonar is directed at them. This means that although sonar can help ships find enemy ships and submarines, it also warns the enemy where the ship sending the sonar is.
Some people believe active sonar may harm animals that live in the sea. Some animals, such as whales and dolphins, use echoes to find food or warn them of danger. It is thought that sonar could confuse these animals and stop them from living their normal lives in the sea.