In other words, and making a parallel with their terrestrial counterparts, a forest in a good conservation state would harbor a great wealth of animals, which produce sounds. Therefore, the diversity and abundance of sounds allow to gather information on species diversity, abundances, and, as a consequence, on the health status of the forest. The same is true underwater: many marine animals produce sound actively (e.g. to communicate and/or find a mate) and passively (e.g. during movement or feeding). Recording these sounds with properly calibrated hydrophones can provide data on the behavior of the species (nutrition, reproduction, etc.), their activity (e.g. resting, swimming, etc.) and their abundance. The advantage of this methodology over direct observation is that hydrophones can collect data without impacting the environment, continuously over consecutive days, so that the presence of nocturnal species can also be investigated. Moreover, there are many species, usually difficult to see and observe, but that emit sounds. This is particularly relevant when studying black coral forests, as they occur at great depths (between 60 and 100 meters deep) and dive times are thus limited.