Acoustic releases are required to retrieve instruments anchored below the ocean surface. For practical applications, dual releases are typically used for safety and redundancy purposes. The releases are placed between the anchor and the moored instruments, and they consist of batteries, an electric motor, and an underwater microphone (hydrophone), all contained inside a high-pressure housing.
If the instruments were to be retrieved after one year, for example, another hydrophone is lowered from the ship at that time, communicating with the releases.
Each release responds to a specific, pre-programmed sound signal. Upon receiving this specific signal, the release separates the mooring from the anchor, and the mooring ascends to the surface. The anchor remains at the ocean floor. Since it is made entirely of iron which occurs naturally in sea water (as are all other chemical elements), it poses no environmental risk to the ocean.