The Earth's lower mantle, extending from 670 to 2890 km depth, is primarily composed of (Mg,Fe)SiO₃ perovskite, (Mg,Fe)O, and CaSiO₃. However, this mineralogy cannot fully explain the unique properties of the D” layer, the lowermost 150 km of the mantle. Using ab initio simulations and high-pressure experiments, Oganov and Ono demonstrate that at the conditions of the D” layer, MgSiO₃ transforms from perovskite into a layered CaIrO₃-type structure (space group Cmcm). This new phase explains several puzzles of the D” layer, including its seismic anisotropy, shear-wave discontinuity, and possible anticorrelation between shear and bulk sound velocities. The predicted seismic signatures match seismological inferences, and the elastic properties of the post-perovskite phase align with experimental findings. The discovery of this phase suggests that it is a major component of the D” layer, potentially affecting mantle dynamics, geochemical anomalies, and the chemistry of plume magmas. The results challenge traditional assumptions and highlight the importance of studying the deep Earth to uncover unexpected phenomena.The Earth's lower mantle, extending from 670 to 2890 km depth, is primarily composed of (Mg,Fe)SiO₃ perovskite, (Mg,Fe)O, and CaSiO₃. However, this mineralogy cannot fully explain the unique properties of the D” layer, the lowermost 150 km of the mantle. Using ab initio simulations and high-pressure experiments, Oganov and Ono demonstrate that at the conditions of the D” layer, MgSiO₃ transforms from perovskite into a layered CaIrO₃-type structure (space group Cmcm). This new phase explains several puzzles of the D” layer, including its seismic anisotropy, shear-wave discontinuity, and possible anticorrelation between shear and bulk sound velocities. The predicted seismic signatures match seismological inferences, and the elastic properties of the post-perovskite phase align with experimental findings. The discovery of this phase suggests that it is a major component of the D” layer, potentially affecting mantle dynamics, geochemical anomalies, and the chemistry of plume magmas. The results challenge traditional assumptions and highlight the importance of studying the deep Earth to uncover unexpected phenomena.