Earth’s Hidden “Inner-Inner” Core Confirmed by Australian Seismologists

In 2023 researchers Thanh-Son Pham and Hrvoje Tkalcic at the Australian National University (ANU) announced the detection of a separate iron-rich sphere inside Earth’s solid inner core. By stacking seismic recordings from roughly 200 giant earthquakes—including the magnitude 9.1 Sumatra event of 2004 and the magnitude 9.0 Tohoku quake of 2011—they isolated waves that traversed the deepest ~650 km of the core. Those waves showed a different anisotropic pattern: the fastest direction for P-waves tilts toward the equatorial plane (east-west) in the innermost region, whereas the surrounding inner-core shell conducts fastest along the spin axis (north-south). The inner-inner core is about 1,300 km in diameter—roughly the width of Texas—and is estimated to weigh on the order of 10²² kg.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inneEarth’s inner core is an iron-rich ball as hot as the surface of the Sun, kept solid only because the crushing pressure of the entire planet raises the temperature at which iron can melt

The solid inner core itself was first inferred by Danish seismologist Inge Lehmann in 1936, who recognized anomalous P-wave arrivals that could not be explained by a wholly liquid core. By the 1980s seismologists noted anisotropy: P-waves travel 3–4 % faster pole-to-pole than equator-to-equator, implying crystal alignment along Earth’s rotation axis. In 2002 Miaki Ishii and Adam Dziewonski (Harvard) proposed an “innermost inner core” where crystals might align differently, but evidence remained ambiguous for two decades.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Thanh-Son Pham – ANU seismologist, co-author of the 2023 study.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Hrvoje Tkalcic – ANU seismologist, co-author of the 2023 study.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Tao Wang and Xiaodong Song – University of Illinois and Nanjing University researchers who, in 2015, first reported east-west crystal alignment in the deepest core using earthquake-coda autocorrelation.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Inge Lehmann – Danish seismologist who discovered the inner core in 1936.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Diameter of innermost core: ~1,300 km.

Earth’s inner core is an iron-rich ball as hot as the surface of the Sun, kept solid only because the crushing pressure of the entire planet raises the temperature at which iron can melt

• Depth range examined: deepest ~650 km of the inner core.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Number of earthquakes stacked: ~200 large events.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

• Anisotropy contrast: fastest P-wave direction shifts from north-south (outer shell) to east-west (inner sphere).

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

The newly identified boundary marks a change in crystal orientation at the atomic scale, implying either a fossilized early-core segment frozen under a different magnetic field or a transition to a body-centred cubic iron phase stable only at extreme pressures. Because inner-core growth releases latent heat and light elements that drive convection in the liquid outer core, the existence of an inner-inner core adds a subtle but permanent source of chemical and thermal heterogeneity. These heterogeneities influence the geodynamo that sustains Earth’s magnetic field, which shields the atmosphere from solar wind and enables surface habitability.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inneThe Earth's inner core grows by roughly a millimetre a year as molten iron freezes onto it, and that slow crystallisation releases the energy that stirs the outer core and keeps the planet's magneticEarth’s inner core is an iron-rich ball as hot as the surface of the Sun, kept solid only because the crushing pressure of the entire planet raises the temperature at which iron can melt

The ANU team described their method as “listening to the planet like a struck bell,” emphasizing that the signal from the deepest core “behaved differently from waves crossing the surrounding shell.” Their findings were presented as a confirmation of the long-standing hypothesis that the inner core contains multiple structural layers.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

Early studies after 2002 produced mixed results: some seismic analyses supported a distinct innermost region, while others found no statistical need for it. The 2023 work resolves much of this disagreement by employing a novel whole-planet reverberation technique, yet the precise crystal phase (hexagonal close-packed vs. body-centred cubic) remains unresolved, and the temporal evolution of the inner-inner core’s size is still unknown.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne

Future research will apply the reverberation stacking approach to newer megathrust events and integrate high-pressure laboratory experiments to test the cubic-iron hypothesis. The August 2024 discovery of a doughnut-shaped anomaly in the outer core suggests that Earth may host additional, as-yet-unidentified layers, prompting a re-examination of the planet’s internal architecture.

In 2023, seismologists at the Australian National University, reading the echoes of earthquakes ringing back and forth through the whole planet, confirmed Earth hides an innermost core inside its inne