Some notable noises: monsters and machines

Against the continuum of the low-frequency ambient sea noise that is due to ships and weather, there appear a variety of 'strange' noises, usually transient in nature. The sources of some of these have been identified as large mammals or large machines; others remain a mystery. A review of the characteristics of a number of these strange sounds is presented. In particular, certain cetacean noises and noises that appear to be associated with offshore oil operations are described.