Understanding planetary structure and evolution requires a detailed knowledge of the properties of geological materials under the conditions of deep planetary interiors. Experiments under the extreme pressure–temperature conditions of the deep mantle are challenging, and many fundamental properties remain poorly constrained or are inferred only through uncertain extrapolations from lower pressure–temperature states. Nevertheless, the last several years have witnessed a number of new developments in this area, and a broad overview of the current understanding of the Earth's lower mantle is presented here. Some recent experimental and theoretical advances related to the lowermost mantle are highlighted. Measurements of the equation of state and deformation behaviour of (Mg,Fe)SiO₃in the CaIrO₃-type (post-perovskite) structure yield insights into the nature of the core–mantle boundary region. Theoretical studies of the behaviour of MgSiO₃liquids under high pressure–temperature conditions provide constraints on melt volumes, diffusivities and viscosities that are relevant to understanding both the early Earth (e.g. deep magma oceans) and seismic structure observed in the present Earth (e.g. ultra-low-velocity zones).