SignificanceNearly three decades ago, seismologists discovered peculiarly dense and slow patches just above Earth’s core−mantle boundary (CMB), known as the ultralow velocity zones (ULVZs). The origin of the ULVZs has remained enigmatic because silicate partial melt models face challenges in accounting for the nonubiquitous occurrence of ULVZs and explaining the observed density excess, whereas iron-rich solid models may have difficulty reproducing the sound velocity observations. Here we propose hypotheses involving slab-derived metallic melt as a critical component to explain the density and velocity features of the ULVZs. These hypotheses differ fundamentally from existing models and may provide insights into the influence of the deep carbon cycle on Earth’s dynamics.