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Ultralow-velocity zones (ULVZs) are regions of the Earth's core–mantle boundary about 1–10 kilometres thick exhibiting seismic velocities that are lower than radial-Earth reference models by about 10–20 per cent for compressional waves and 10–30 per cent for shear waves. It is also thought that such regions have an increased density of about 0–20 per cent (ref. ). A number of origins for ULVZs have been proposed, such as ponding of dense silicate melt, core–mantle reaction zones or underside sedimentation from the core. Here we suggest that ULVZs might instead be relics of banded iron formations subducted to the core–mantle boundary between 2.8 and 1.8 billion years ago. Consisting mainly of interbedded iron oxides and silica, such banded iron formations were deposited in the world's oceans during the late Archaean and early Proterozoic eras. We argue that these layers, as part of the ocean floor …