The U.S. government is developing a new way to keep tabs on enemy submarines. It doesn’t involve flashy technology; instead, officials hope to rely on aquatic creatures to alert them to potential danger. The military’s research arm, the Defense Advanced Research Projects Agency (DARPA), recently began studying how marine animals react to changes in the water. Goliath grouper fish, for example, make a booming sound when divers swim by, while some micro-organisms light up in response to touch and movement. If scientists could understand those signals more precisely, it would enable the military to monitor a wider area using fewer resources. “Mostly people think of those sounds as background noise,” says program manager Lori Adornato. “Maybe there’s more value to those sounds, along with other biological signals, and if we learn how to understand them, then we’ll be strategically better off.”