Put your ear to the ground in the right place and you can hear a train coming from miles away. Long before it appears on the horizon, the vibrations travelling through the earth betray its approach. Scientists have been doing something remarkably similar with the Sun, pressing their ear metaphorically speaking, to the solar surface and listening to the rumbles coming from deep within. What they've just heard has them genuinely puzzled. The technique is called helioseismology, and it works by tracking tiny sound waves that reverberate around inside the Sun. Just as geologists use seismic waves to map Earth's interior, solar physicists use these oscillations to peer beneath the Sun's visible surface, somewhere no instrument could ever directly reach. And over the past forty years, a global network of six telescopes called BiSON, the Birmingham Solar Oscillations Network, has been quietly listening, building up one of the most remarkable datasets in all of astrophysics.…