On December 20, Hunga Tonga-Hunga Ha’apai—an underwater volcano in the South Pacific topped with a diminutive and uninhabited island—awoke from a seven-year slumber. The volcano spluttered and crackled, creating a large plume of ash. Ten thousand miles away, in England, Simon Proud, a satellite data researcher at the University of Oxford, began to monitor the twitching volcano using an array of...