Sonar, an acronym for SOund NAvigation (and) Ranging, is a technique that uses sound propagation to communicate with or detect objects on or under the surface of a body of water. The first recorded use of the technique was by Leonardo da Vinci in 1490 via a tube inserted into the water to detect vessels by ear. Sonar equipment was developed during World War I to counter the growing threat of submarine warfare; an operational passive sonar system, which only listens for the sound of vessels, was in use by 1918. Modern active sonar systems use an acoustic transponder to generate a sound wave which is then reflected back from target objects.
This picture shows a sonar image of the Soviet Navy minesweeper T-297, formerly the Latvian Virsaitis, that was shipwrecked on 3 December 1941 in the Gulf of Finland.Image: Tuukritööde OÜ