The book, designed for a wide circle of readers and is essentially the second, revised and supplemented edition of the book "Underwater Sound" (Sudpromgiz, 1963), is devoted to the rapidly developing field of science and technology - hydroacoustics. In a lively popular form, the author introduces the reader to the main physical phenomena associated with the propagation of sound in the sea, such as marine reverberation, refraction, absorption of underwater sound, its reflection from marine objects.