The Sun runs on an 11-year cycle of rising and falling activity, tracked mainly by counting sunspots – the dark patches scattered across its surface. Solar Cycle 25, the current cycle, was forecast as mild, and the sunspot count agreed. But those counts read only the surface. A global network of six telescopes has listened to the Sun’s interior for nearly 40 years, and what it is now telling researchers is not what the surface suggested at all. Listening inside the Sun Scientists have a name for eavesdropping on those sound waves: helioseismology. The waves are trapped inside the Sun,...