We have assembled a data set of 302 differential PKiKP-PcP travel times that were recorded by seismic array stations of the International Monitoring System. This is an order of magnitude more data than all previous PKiKP-PcP observations combined. The differential times were measured using a beam-forming and cross-correlation algorithm that has a precision of 1--2 sampling intervals (<0.1 s). The use of differential times mitigates the effects of hypocentral errors and heterogeneities in the crust and upper mantle, leaving the data sensitive to deep Earth structure. The PKiKP-PcP times are especially sensitive to variations in the thickness of the liquid outer core. The data are well fit by PREM, with residuals having a mean of -0.29 s and a standard deviation of 0.44 s, and rule out lateral variations in outer core thickness of more than 4 km. Most of the geographically coherent residuals can be explained by lower mantle heterogeneities and ellipticity corrections however there remain at least three regions that seem to require aspherical core structure. The anomalous data are best explained by long-wavelength topography on the core-mantle boundary (CMB) with peak-to-peak amplitude of 3.5 km. The CMB topography is most likely maintained by isostatic compensation of thickness variations in a distinct layer at the base of the mantle.