New images of the three-dimensional structure of Earth's upper mantle revealed by large-scale processing of global seismic data are beginning to change our understanding of convection in the planet. The processing techniques combine data from hundreds of seismograms to distinguish weak features from the background noise. These procedures are termed “stacking” and are similar in many respects to reflection seismic techniques long used in the oil industry to image shallow crustal structure. However, the new results extend much deeper and map reflections from several internal discontinuities—interfaces separating different mantle layers—within the upper 700 km of the mantle. Observed deflections and topography on these discontinuities provide important constraints on the nature of the long-term deformation of the mantle. The mantle is the 2900-km-thick shell surrounding Earth's core. It is solid but can slowly flow and overturn at the high pressures and temperatures present within Earth's interior.