Abstract Due to the efficient propagation of sound in water, sound in the deep ocean propagates such great distances that soundscapes are influenced not only by local conditions but also by distant sound sources. Ocean Sound is now an Essential Ocean Variable within the Global Ocean Observing System making passive acoustic monitoring routine. Active acoustic probing of the environment informs us about ocean topography, currents and temperature, and abundance and type of marine life vital to fisheries and biodiversity related interests.Efficient sound propagation is the foundation of a proposed multipurpose acoustic network. Judiciously placed low-frequency acoustic sources transmitting to globally distributed passive acoustic systems provide: (1) high temporal resolution measurements of large-scale ocean temperature/heat content variability using tomography; and (2) underwater geo-positioning (UW-GPS) and communication services enabling basin-scale underwater operation of floats, gliders, and AUVs. Every platform (fixed or moving) equipped with a hydrophone becomes a “GPS” receiver, while listening to the ocean soundscape. The combined active and passive acoustic technology will lead to multi-disciplinary discovery and improved understanding of ocean ecosystem health and biodiversity, climate variability and change, marine hazards, and maritime safety. The same system will improve the operation of gliders, floats and AUVs.