Former CDAC student Wendy L. Mao (now at Los Alamos National Laboratory) and colleagues from the Geophysical Laboratory and the Advanced Photon Source (including HPCAT, XOR, and GSECARS) report a major new finding in high-pressure geophysics. The work focuses on iron-rich post-perovskites and the ultralow-velocity zones in a paper in the April 28th issue of Science. The boundary layer between the crystalline silicate lower mantle and the liquid iron core contains regions with ultralow seismic velocities. Such low compressional and shear wave velocities and high Poisson's ratio are also observed experimentally in post-perovskite silicate phase containing up to 40 mol% FeSiO₃ endmember. The iron-rich post-perovskite silicate is stable at the pressure-temperature and chemical environment of the core-mantle boundary and can be formed by core-mantle reaction. Mantle dynamics may lead to further accumulation of this material into the ultralow-velocity patches that are observable by seismology.