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Three source regions show evidence of a low-velocity layer that is less than 15 kilometers thick on top of the core-mantle boundary and require about a 3:1 ratio of shear-to-compressional velocity reduction, which is consistent with partial melt. Layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity. These observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core-mantle boundary. Low viscosity accompanying partial melt can localize the upwelling of warmed mantle, making the low-velocity layer a plausible source of mantle plumes.