ABSTRACT
The sound of an outboard motor can put a diver into a state of high alert, aware that a trip back to the surface is fraught with the danger of being struck by a boat. It is impossible for us to judge the direction, or distance to a sound source underwater: the normal cues we use to localize sound in air simply do not work underwater. In addition, much of the sound we hear is from the operation of our own demand regulator. Holding our breath to listen raises the risk of lung embolism if we are ascending. But if we safely stop and listen we can hear the sounds of breaking waves, boat propellers, grunting and chewing fish, snapping shrimps, whale song, and dolphin calls. For the creatures that inhabit the sea, these sounds are ideal for communication and navigation.
