Scattered waves that precede the seismic phase PKP (which traverses the Earth＇s core) have been used to identify and locate small-scale heterogeneity in the Earth＇s mantle(1-6). A recent study has demonstrated that the global data set of these precursors is consistent with weak heterogeneity (about 1 per cent r.m.s. velocity variation) distributed throughout the mantle(7). Here we show, however, that anomalously large PKP precursors from earthquakes in northern Tonga require much stronger heterogeneity (10-15 per cent r.m.s. velocity variation) in a layer about 60 km thick near the core-mantle boundary below Tonga. This region of the core-mantle boundary is also marked by low shear-wave velocities in the lower mantle(8) and is near an area of very low compressional-wave velocity in the lowermost tens of kilometres of the mantle(9), which has been interpreted as evidence for the presence of partial melt(10). The strength of the scattering that we observe provides strong support for the presence of partial melt in this area, and also suggests that vigorous small-scale convection is taking place at the core-mantle boundary.