In 1991, scientists lowered large subwoofers into the water at Heard Island, a snowcapped volcanic island in the Indian Ocean. The speakers emitted low-frequency sounds that, like whale song, rumbled across entire oceans. Picked up by receivers off the coasts of California and Bermuda, the signals contained a crucial piece of information about the water they had traversed: how hot it was. It was a promising way to monitor Earth’s warming oceans, but concerns about how the underwater noise might affect marine life soon sidelined it, with only a few dedicated scientists keeping the technique alive. Now, it is back—only this time, Earth itself is providing the noise.