Deep within Earth lies a solid metal core that spins independently of our planet's rotation, akin to a top spinning inside a larger top. This enigmatic inner core has fascinated scientists since Danish seismologist Inge Lehmann discovered it in 1936. Researchers have long debated the core’s rotation speed and direction. Recent evidence suggests significant changes in its spin, but consensus remains elusive due to the inability to directly observe or sample Earth's deep interior.

Seismologists derive insights into the core’s motion by analyzing how seismic waves from large earthquakes interact with it. By comparing similar waves over time, they can measure changes in the core's position and calculate its rotation. Differential rotation of the inner core was proposed in the 1970s and 1980s, but seismological evidence only emerged in the 1990s. However, interpretations of these findings vary, leading to disagreements on the core’s rotation rate and direction. Some studies even suggest the core might not rotate at all.

A 2023 model posited that the inner core, once spinning faster than Earth, had slowed down. Initially matching Earth's spin, it eventually slowed further, moving backward relative to the surrounding fluid layers. This hypothesis required more data for validation. A June 2023 study published in *Nature* provided new evidence supporting this model, indicating the core's rotational speed follows a 70-year cycle. Study coauthor Dr. John Vidale of the University of Southern California stated this research resolves the debate about the core's movement patterns over recent decades.

Despite this, some experts remain unconvinced, and the implications of the core's slowdown for Earth are still unclear, though some suggest it could impact the planet's magnetic field. The inner core, located about 3,220 miles (5,180 kilometers) deep, is a hot, solid metal ball primarily composed of iron and nickel. Its temperature is estimated to match the sun’s surface at around 9,800 degrees Fahrenheit (5,400 degrees Celsius). Earth's magnetic field, which shields the planet from harmful solar radiation, influences the core’s spin. Simultaneously, the gravity and flow of the outer core and mantle exert drag on the core, leading to variations in its rotational speed.

The outer core’s fluid movement generates electrical currents that power Earth’s magnetic field. Although the inner core’s direct influence on the magnetic field is uncertain, a slower-spinning core might affect it and slightly shorten the length of a day. Seismologists track seismic waves—pressure waves (P waves) and shear waves (S waves)—to study Earth's interior. P waves traverse all matter types, while S waves only move through solids or highly viscous liquids. Observations of S waves led to the conclusion that Earth’s core is molten, while anomalies in P waves suggested the presence of a solid inner core, a theory first proposed by Lehmann.

The 2023 study analyzed seismic waves from earthquakes passing through the inner core since 1964, revealing a 70-year rotational cycle. The core spun slightly faster than Earth in the 1970s, slowed around 2008, and then began moving in reverse relative to the mantle. Vidale and colleagues examined seismic waves from repeated earthquakes in the South Sandwich Islands and Soviet nuclear tests, confirming changes in core rotation and the 70-year cycle.

Their findings suggest the core is about to start speeding up again, though the varying rates of acceleration and deceleration within the cycle require further explanation. One theory is that the inner core might not be as solid as expected, with potential deformation affecting rotational symmetry. Different rotation rates for forward and backward motion add complexity to the discussion.

Despite these findings, uncertainties persist due to the core’s inaccessibility. More data and interdisciplinary tools are needed for further investigation. Changes in core spin are imperceptible to humans, only affecting day length by mere thousandths of a second. Understanding the inner core’s behavior is crucial for insights into Earth's formation and the interactions across its subsurface layers. The boundary between the liquid outer core and the solid inner core, along with the core-mantle boundary, is particularly intriguing for potential activity, including hypothetical volcanoes at these interfaces.

Inner core rotation likely influences Earth’s magnetic field, but more research is needed to clarify its role. Novel methodologies will be essential to answer ongoing questions about the inner core, including its rotation dynamics.