Between a rock and a hot place: the core-mantle boundary.

The boundary between the rocky mantle and iron core, almost 2900 km below the surface, is physically the most significant in the Earth's interior. It may be the terminus for subducted surface material, the source of mantle plumes and a control on the Earth's magnetic field. Its properties also have profound significance for the thermochemical and dynamic evolution of the solid Earth. Evidence from seismology shows that D'' (the lowermost few hundred kilometres of the mantle) has a variety of anomalous features. Understanding the origin of these observations requires an understanding of the elastic and deformation properties of the deep Earth minerals. Core-mantle boundary pressures and temperatures are achievable in the laboratory using diamond anvil cell (DAC) apparatus. Such experiments have led to the recent discovery of a new phase, 'post-perovskite', which may explain many hitherto poorly understood properties of D''. Experimental work is also done using analogue minerals at lower pressures and temperatures; these circumvent some of the limits imposed by the small sample size allowed by the DAC. A considerable contribution also comes from theoretical methods that provide a wealth of otherwise unavailable information, as well as verification and refinement of experimental results. The future of the study of the lowermost mantle will involve the linking of the ever-improving seismic observations with predictions of material properties from theoretical and experimental mineral physics in a quantitative fashion, including simulations of the dynamics of the deep Earth. This has the potential to dispel much of the mystery that still surrounds this remote but important region.