Stacks of global seismic data reveal the time and distance dependence of high-frequency (1-hertz) precursors to the seismic phase PKKP. Synthetic seismogram modeling shows that scattering from random small-scale topography at the PKKP core-mantle-boundary reflection point generates precursory arrivals similar to those seen in the data. Models with a root mean square core-mantle-boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data. However, a systematic range-dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors.