For the past decade, a suite of devices have been monitoring Axial’s every action — rumbling, shaking, swelling, tilting — and delivering real-time data via a seafloor cable. It’s “the most well-instrumented submarine volcano on the planet,” says Mark Zumberge, a geophysicist at Scripps Institution of Oceanography in La Jolla, Calif., who was not involved in the work. But in November, a particular milestone caught Chadwick’s eye: Axial’s surface had ballooned to nearly the same height as it had before its last eruption in 2015 — fortuitously, just months after monitoring began. Ballooning is a sign that magma has accumulated...