How can submarines detect other underwater objects?

Underwater objects are usually tracked by means of the sounds given off by an object. For a submarine, sound is generated largely by its propellers as they churn through the water. However, the engines and some other parts also generate noise. These noises are detected by a submarine's sonar (sound detection and ranging equipment).

Objects can also be located by reflected sound waves, known as "active sonar". A "ping" or sound wave is generated into the water. Any differences in the returning sound indicates the location of an object or structure underwater. This technique can also be used to map the ocean floor.