Hydrophone arrays have been developed to detect and monitor previously unheard volcanic and earthquake activity in the oceans and in sea waves. Hydrophones are connected by undersea communication cables to facilities on seaside. Individual hydrophone array are installed essentially on continental slopes and seamounts at locations that are unobstructed by seafloor features that would block sound wave transmission across large areas of the oceans. There are many ways to design hydrophones. We can design hydrophone with adequate broadband sensitivity. The hydrophones are small, lightweight, and inexpensive compared to the high-power projectors needed for active sonars. The hydrophone arrays are used to locate submarines.To track marine mammals and even to study global climate change by for example detecting temperature differences hydrophone arrays used.