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An introduction to echoes and how they are used in sonar to locate objects underwater. The presenter explains how sound travels through solids and liquids as particles vibrate. However, when sound comes into contact with hard flat surfaces, some of it is reflected – this is called an echo. Sonar (originally an acronym for 'Sound Navigation and Ranging') is a sophisticated technique using this principle, and is used at sea to locate the sea bottom and other large objects in the water.