Radio waves do not travel well underwater. That is why ships employ sonar rather than radar to plumb the briny depths. Messages broadcast through the ocean need to be sonic, too. For that purpose people often use acoustic modems, which can turn electronic signals into sound, and vice versa, like an old-fashioned acoustic coupler for a telephone.
Such instruments need power, though. And if they are sitting on the seabed, replacing their batteries is a serious chore. But Fadel Adib of the Massachusetts Institute of Technology (MIT) may have the answer. A device he has created and tested not only broadcasts and receives sound—it is powered by sound as well.