The ocean is vast, so how do researchers at national marine sanctuaries study what lives there? They listen! The sounds fish and other marine organisms make are species-specific, so listening to them is a great way to determine what’s in the sanctuary. Researchers at Stellwagen Bank National Marine Sanctuary, Gray's Reef National Marine Sanctuary, Florida Keys National Marine Sanctuary, and Flower Garden Banks National Marine Sanctuary deploy shallow water hydrophones to record ambient sound, like that of fishes and boats. Researchers have also deployed deep-water hydrophones in Cordell Bank National Marine Sanctuary, Channel Islands National Marine Sanctuary, and Olympic Coast National Marine Sanctuary to record sounds like whales and ships. These listening stations will help us learn how noisy our sanctuaries are.