Acoustic techniques have a long history of use in the sea. As man becomes more involved in this major portion of his environment, his use of acoustics increases both in quantity and variety. A number of acoustic devices played key rôles in the search for the lost submarines,THRESHER(SSN 593) andSCORPION(SSN 589), as well as in the location and recovery of the nuclear device lost off the coast of Spain. These mishaps have spurred research and development of acoustic and other types of equipment for use on the deep ocean floor and, while considerable progress has been made, there remains much room for improvement. As a result of its successful participation in these search efforts, the Naval Research Laboratory has become deeply involved in man's quest for mastery over this hostile portion of his habitation.