We explore lowermost mantle structure beneath the Pacific with long-period recordings of the seismic core phases SKS, SPdKS, and SKKS from 25 deep earthquakes. SPdKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SPdKS arrivals are explained by a laterally varying mantle-side boundary layer at the CMB, having P-velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large-scale low velocity feature in the lower mantle beneath the central-Pacific. SKS, SPdKS, and SKKS data for the generally faster-than-average circum-Pacific lower mantle are well-fit by models lacking any such low-velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions. ¿ American Geophysical Union 1996