Ocean acoustic tomography is a computerized tomography (CT) system, which uses sound waves to provide ocean conditions instantly over a surface area three times larger than Japan. In seawater, light and electric waves travel relatively short distances. Sound, however, carries much farther than in air. Ocean phenomena such as the Kuroshio Current play an important role in the energy flux of the sea. When this CT is used for studying such large-scale ocean movements, we can get detailed observations of ocean conditions, previously unclear or unobtainable. Much of our current efforts are being devoted to the development of systems capable of observing large areas of the ocean instantly. Advantages Offered by the New Information Ocean Acoustic Tomography In less than 10 minutes an area of the ocean three times larger than Japan can be examined, from the surface to the bottom. A weather map of the sea can be drawn up instantly for phenomena such as the Kuroshio, cold water and warm water masses. By following ocean changes over time, we can gain a systematic understanding of the relationship between the atmosphere and the ocean. It will provide useful data for checking global atmosphere-ocean models and for simulations, which are expected to contribute to the understanding of changes in the atmosphere and the ocean.