Sonar is equipment on a ship or a submarine that uses sound to find things under the water. It sends out a sound — the famous 'ping' you hear in submarine films — and waits for the echo to come back. From how long the echo takes and which direction it comes from, the machine works out where an object is, how far away, and often how big. That is called active sonar. There is also passive sonar, where you send nothing at all and only listen, because sending a ping tells the enemy exactly where you are. Sonar is not only for war: fishing boats use it to find fish, and ships use it to measure how deep the water is. Animals do the same trick — bats and dolphins — and people often call that natural sonar, though the proper word is echolocation. The word also gets used loosely for noticing things: 'that never showed up on my sonar' means I never became aware of it, though for that meaning most people say radar instead.