AT the base of the Earth’s mantle is a region, called D ", which serves as a thermal and chemical boundary layer between the silicate mantle and the liquid-iron outer core(1,2). Tomographic models of mantle compressional-wave velocity (nu(P)) have their worst resolution in D ", owing to limited ray sampling(3-5), but have hinted at large-scale lateral variations there. Here I use a new technique, which has its greatest resolution within D ", to produce a map of the large-scale nu(P) variations within D ". The technique compares the arrival times of waves that have been refracted across and diffracted around the core-mantle boundary (CMB). The diffracted waves travel a long way in D ", making them excellent probes of this layer, and the differential technique removes many effects arising from ray paths outside of D " (ref. 6). The new map provides an image of the continent-sized variations at the CMB that is complementary to existing shear-wave data(7,8), and similarities between the map and projections of ancient subducted lithosphere(9) verify previous suggestions(10) of a strong coupling between surface plate tectonics and the base of the mantle.