Seismic observations have revealed multi-scale seismic heterogeneities in Earth’s lowermost mantle, while advances in mineral physics, geochemistry, and experimental petrology combined with geodynamic modeling have greatly enhanced our knowledge of these deep mantle structures. However, despite great progress, the detailed structure, dynamics, origin, and evolution of the lowermost mantle seismic heterogeneities remain unclear, such as those of the ultra-low velocity zones (ULVZs), the large-low shear velocity provinces (LLSVPs) and the D" layer. What are the internal structures of these seismic anomalies? To what extent are they caused by thermal or compositional effects? How do they interact with the surrounding mantle? How are they related to the processes of crystallization of a basal magma ocean, core–mantle interactions, subduction of oceanic crust, and giant impacts? The session invites contributions that provide new constraints on seismic observations, chemical and physical processes of Earth’s lowermost mantle and their role in the whole Earth system.