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The conventional explanation of the formation of the solid inner core involves slow cooling and crystallization. Because the melting temperature increases with pressure, the core will solidify from the center outwards. But this effect also means that as pressure increases because of accretion, the core can pressure-freeze when the Earth reaches a critical size, unless there is a large amount of superheat. Although we know that the magnetic field is ancient and that a solid and growing inner core may be essential to its existence, it is possible that catastrophic events such as the Moon-forming impact may have caused the inner core to reform one or more times. Initial superheat and episodic growth will possibly resolve some of the current energy problems (ref. 20, and D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work). A growing inner core is needed to power the current dynamo, but rapid cooling may have powered the ancient dynamo (D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work). The inner core may, therefore, be much younger than the Earth. The heterogeneity and anisotropy of the inner core may help constrain its apparently complex history.