Broadband seismic stacking is performed for PcP waves sampling the lowermost mantle and core-mantle boundary beneath central Mexico and the western Gulf of Mexico. The data contain evidence of a 10to 20-km-thick ultralow velocity zone on the western edge of the study area, tapering (or vanishing) to the east to less than 5-km thickness over a horizontal distance of ∼200 km. Epicentral distance varies from 20° to 60°, a range over which the amplitude of reflections from the top of the ultralow velocity zone responds strongly to changes in the ratio of shear to compressional velocity reduction (τ = δ ln νs/δ ln νp) and to density within the zone. Where the zone is present and thick enough to separate PcP and the precursory reflection, our modeling is consistent with a value of τ near 3 and δ ln νp near 10%, assuming only a minor density increase (1 to 2%). The value of τ is reduced to 2 for a ∼14% density increase, but δln νp remains large (7%). Very low amplitude multiply converted phases are synthetically well reproduced with extreme attenuation within the zone, suggesting the presence of a melt component.