Sonar

If you haven't seen the movie U-571, this sound shouldn't freak you out too much. Sonar stands for "sound navigation and ranging". It is a technique that uses sound propagation (usually underwater) to navigate and to communicate with/detect other vessels. There are many interesting civil, commercial, and military applications for sonar, including the detection of frogmen (someone who is trained to dive or swim in a military capacity which can include combat). Pretty cool.