Abstract: This invention relates to a secure marine communication system and especiy to a sonic communication system utilizing the so-called "sound channel" in the ocean deeps.The propagation of sound in sea water is known to occur along rays which are subject to refraction by variations in sound velocity. In most areas of the oceans, the velocity of sound varies with depth and temperature, and is less at some intermediate depth than it is either at the surface or at the ocean bottom. These oceanographic conditions produce a "sound channel" having at/its horizontal axis a level along which the velocity of sound reaches its minimum value. Essentially, the sound channel provides for a long-range transmission of sound between a sound source and a receiver where both types of sonic equipment are located at a level approximately corresponding to the axis of minimum velocity.