We know that most magma originates in the Earth’s mantle. As it pushes up through the many kilometers of lithosphere to the surface, it pauses in one or more magma chambers or partially melted mush zones for periods of up to a few millennia before erupting. But while we have seismic evidence and models and support this picture, we have not hitherto been able to watch how magma actually moves in the upper mantle and crust. Bob White has set out to change that. Using a dense array of seismometers, he has been able to pinpoint thousands of tiny earthquakes that reveal the detailed movement of melt through the thick crust of Iceland just before it erupted. White combines this seismic data with geochemical analyses of the lava that can tell us about the depths at which the melt is formed. White is Emeritus Professor of Geophysics in the Department of Earth Sciences at the University of Cambridge.