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 10 to 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 (&tgr;=Δ ln vs/Δ ln vp) 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 &tgr; near 3 and Δ ln vp near 10%, assuming only a minor density increase (1 to 2%). The value of &tgr; is reduced to 2 for a ~14% density increase, but Δ ln vp 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. ¿ 2001 American Geophysical Union