Recent seismological investigations reveal the presence of highly anomalous structures at the base of the mantle, modeled as patches ≤5--50 km thick having ultralow-velocities (-ΔVP~10--20%, -ΔVS~10--50%). Waveform modeling shows seismological data are compatible with the patches exhibiting a wide range of density increases, up to Δ&rgr;~60%, which can be ascribed to chemical contamination of the deep mantle by the core. Not all anomalies require lowermost mantle partial melting, and may be located just below or right at the core-mantle boundary (CMB): a ~1--3 km thick zone of finite rigidity (crystallization?) at the top of the outer core or, more generally, of gradational properties across the CMB can also explain observations. Fuzzy patches at the boundary may be zones of intense chemical and physical interactions between the mantle and core. ¿ 2000 American Geophysical Union