he growth of Earth’s solid inner core from the liquid outer core is central to the dynamics of our planet’s deep interior. This growth generates thermal and chemical buoyancy which is crucial for generating the geomagnetic field. However, the classical view of how the inner core first formed does not consider the physical requirement that liquids must be supercooled below the melting point before freezing can begin. Mineral physics calculations have suggested that at least 450 K of supercooling is needed to spontaneously nucleate the inner core. However, when satisfying inferences from geophysical constraints, the maximum available supercooling has been estimated at 420 K, meaning that the origins of the inner core are enigmatic. We explore the consequences of supercooling the Earth’s ancient liquid core on inner core formation, growth and dynamics, and the interpretation of seismic and palaeomagnetic observations. Collectively, these additional constraints suggest that the core was supercooled less than 100 K at the time of inner core nucleation. Even with small supercooling, Earth’s core is required to have grown rapidly immediately after nucleation. Evidence for this rapid growth may exist in the palaeomagnetic record and seismological signatures of inner core structure.