Earth's inner core
Multiple layers

In 2002, M. Ishii and A. Dziewoński presented evidence that the solid inner core contained an "innermost inner core" (IMIC) with somewhat different properties than the shell around it. The nature of the differences and radius of the IMIC are still unresolved as of 2019, with proposals for the latter ranging from 300 km to 750 km.

A. Wang and X. Song proposed, in 2018, a three-layer model, with an "inner inner core" (IIC) with about 500 km radius, an "outer inner core" (OIC) layer about 600 km thick, and an isotropic shell 100 km thick. In this model, the "faster P wave" direction would be parallel to the Earth's axis in the OIC, but perpendicular to that axis in the IIC. However, the conclusion has been disputed by claims that there need not be sharp discontinuities in the inner core, only a gradual change of properties with depth.

In 2023, a study reported new evidence "for an anisotropically-distinctive innermost inner core" – a ~650-km thick innermost ball – "and its transition to a weakly anisotropic outer shell, which could be a fossilized record of a significant global event from the past." They suggest that atoms in the IIC atoms are slightly differently than its outer layer, causing seismic waves to pass through the IIC at different speeds than through the surrounding core (P-wave speeds ~4% slower at ~50° from the Earth’s rotation axis).
detectable_content:postscript, change_case:capital_word_frequency[3]