The generation of oceanic sound by natural physical mechanisms is a topic of scientific inquiry with a wide range of applications, both environmental and naval. Sound is generated by waves interacting, by waves breaking, by wind noise transmitted directly into, and by rain, snow and spray falling onto the water. Sound is also generated in frozen seas by ice either rubbing or cracking. This book contains the proceedings of an international conference Natural Physical Sources of Underwater Sound held at the University of Cambridge in July 1990. The contents of the 54 papers cover the topics of ambient noise, very low and seismic noise, noise from turbulence and bubbles singly and collectively, rain noise, ice noise, as well as thunder, cosmic ray and sea-bottom saltation. The material represents the considerable advances made by hydrodynamicists and acousticians since the first meeting on the topic held in Lerici, Italy in 1987, and published as a companion volume from Kluwer, entitled Sea Surface Sound . The material in both books is dedicated to characterizing and understanding natural, as opposed to man-made, mechanisms of underwater sound generation. Questions of propagation and scattering are included only as necessary to understanding generation itself. A reader interested either in a review of the status of this interdisciplinary field of geohydrodynamical acoustics, or with a general interest in natural acoustics, will find this book of great value.