The production, propagation, reflection, scattering, and reception of sound in seawater. The sea covers approximately 75% of the Earth's surface. In terms of exploration, visible observation of the sea is limited due to the high attenuation of light, and radar has very poor penetrability into salt water. Because of the extraordinary properties that sound has in the sea, and because of some of the inherent characteristics of the sea, acoustics is the principal means by which the sea has been explored.