Our oceans are naturally noisy places. Wind, waves, animals and humans all contribute to ocean soundscapes. Because visibility is limited and sound travels great distances underwater, many marine animals rely on sound to communicate. However, our increasing use of oceans, including for shipping, mining, recreational boating and construction, adds another layer of noise that may adversely affect marine ecosystems. Listening in on ocean soundscapes using a network of underwater microphones (hydrophones), in a process called Passive Acoustic Monitoring, enables us to monitor the health of coral reefs, study the behaviour of marine animals such as whales and dolphins, and assess the impacts of noise pollution.