At a depth of about 2900 km, the solid silicate rock of Earth's mantle meets the liquid iron alloy of the core. This region, called the core-mantle boundary (CMB), has long been pictured as a simple dividing zone. Recently, however, this neat model has been directly challenged by a broad range of discoveries. These include exciting new evidence for a deep-mantle phase change as reported by Murakami et al. on page 855 of this issue, layering in the lowermost mantle, partial melting, mineral anisotropy, and small-scale convection with formation of whole-mantle plumes. Because of these findings, researchers have created a new paradigm, in which deep-mantle layering and heterogeneity exist globally, with notable regional variations. This heterogeneity is intimately coupled to important processes of the interior, such as modulating heat flow out of the core, and hence fluid core convection currents, and the magnetic field (particularly during reversals). The CMB may also act as a repository for lighter elements that emerge from the core fluid . These phenomena are all consequences of the largest absolute temperature and density contrasts within the planet. Although many of the details of this new paradigm remain enigmatic, the growing body of evidence suggests that the deepest mantle and CMB contain the same degree of chemical, dynamical, structural, and thermal complexity as that of Earth's surface, and hence likely hold important clues for deciphering the evolution and present state of the entire interior.
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