Seismic images of Earth's interior reveal two massive anomalous zones at the base of the mantle, above the core, where seismic waves travel slowly. The mantle materials that surround these anomalous regions are thought to be composed of cooler rocks associated with downward advection of former oceanic tectonic plates. However, the origin and composition of the anomalous provinces is uncertain. These zones have long been depicted as warmer-than-average mantle materials related to convective upwelling. Yet, they may also be chemically distinct from the surrounding mantle, and potentially partly composed of subducted or primordial material, and have therefore been termed thermochemical piles. From seismic, geochemical and mineral physics data, the emerging view is that these thermochemical piles appear denser than the surrounding mantle materials, are dynamically stable and long-lived, and are shaped by larger-scale mantle flow. Whether remnants of a primordial layer or later accumulations of more-dense materials, the composition of the piles is modified over time by stirring and by chemical reactions with material from the surrounding mantle, underlying core and potentially from volatile elements transported into the deep Earth by subducted plates. Upwelling mantle plumes may originate from the thermochemical piles, so the unusual chemical composition of the piles could be the source of distinct trace-element signatures observed in hotspot lavas.