We present the first waveform migration for the lower mantle, focusing on the lowermost 400 km of the mantle beneath northern Siberia. We migrate vertical component P wave seismograms recorded in western Europe for earthquakes located in the Kuril Islands region. We focus on the time window between the direct P wave and the PcP wave, reflected at the core-mantle boundary, and we investigate the origin of PcP precursors. The data, selected for their good signal-to-noise ratio, are deconvolved by the source time function determined by the beam of the P wave. We observe the signature of a finite size scattering object located around 75 ¿N, 85 ¿E, at an apparent depth of ~2650 km with respect to the IASP91 model. Its scale length is probably of the order of 100 km. This object is characterized by an anisotropic scattering diagram and may be related to the presence of an ancient subducted plate near the core-mantle boundary. ¿ 2001 American Geophysical Union