Earth’s Inner Core Just Had Its Busiest Discovery Stretch Yet

Roughly 5,150 kilometers beneath your feet sits a solid ball of iron-rich metal, smaller than the Moon but probably heavier, and still growing by about a millimeter a year. No drill has ever touched it. Everything scientists know about it comes from earthquake waves, lab experiments that recreate crushing pressure for a fraction of a second, and models of how iron behaves when squeezed harder than anywhere else on the planet.

That tidy picture is getting more complicated by the month. Fresh research says the core may hold a second, stranger sphere inside it, that its shape is shifting at the boundary with the liquid layer above it, and that its age is disputed by hundreds of millions of years, a gap that changes how long Earth’s magnetic shield has actually been running.

A Metal Sphere Smaller Than the Moon, Heavier All the Same

The standard map of Earth’s interior comes from the Preliminary Reference Earth Model, published in 1981 by seismologists Adam Dziewonski and Don Anderson. It places the boundary between the liquid outer core and the solid inner core about 5,150 kilometers down, with the inner core extending roughly 1,220 kilometers more to the planet’s center.

That radius makes the inner core about 70% the size of the Moon, a comparison CNN drew when covering recent core research. Size is not the same as heft. Iron-rich metal under that much pressure runs far denser than rock, so the sphere likely outweighs the Moon despite being smaller across.

Its exact recipe is still argued over. A 2014 paper in the Proceedings of the National Academy of Sciences by James Badro, Alexander Cote and John Brodholt modeled the core as iron alloyed with nickel plus lighter elements such as oxygen, silicon, sulfur or carbon. Small changes to that mix shift density, melting behavior and how fast seismic waves move through it.

How Seismic Waves Reach a Place No Drill Can Touch

No instrument has ever visited the inner core, and no sample has ever come back from it. Humans have traveled roughly 25 billion kilometers into space, yet the deepest borehole ever drilled has barely passed 12 kilometers into the crust, according to a University of Liverpool study on deep-Earth structures.

So geophysicists rely on indirect evidence instead:

• Seismic waves that behave differently in solids than in liquids, mapped from earthquakes recorded around the world
• Mineral physics experiments that compress iron alloys to core-like pressures inside a lab
• Density models built from Earth’s total mass, gravity field and rotation
• Calculations of how iron alloys freeze and flow under conditions no lab can fully hold for long

Researchers at Lawrence Livermore National Laboratory pushed that lab approach further this month, using the National Ignition Facility’s lasers to compress iron and reach pressures of 3 million atmospheres while heating it to 5,000 degrees Celsius without letting it melt too fast to measure. \u201cWe study iron because it is a primary constituent of Earth’s and other terrestrial planet cores, and how it functions under inner-core conditions is not well understood,\u201d said Yong-Jae Kim, an LLNL physicist and co-lead author of the resulting Nature Communications paper.

Why the Core Keeps Freezing

As Earth loses heat through its mantle and crust, the liquid outer core cools until iron-rich metal crystallizes and plates onto the solid sphere below it. That single process does two jobs. It makes the inner core bigger, and it leaves lighter elements behind in the liquid above, which helps drive the churning that generates Earth’s magnetic field.

A 2012 Nature paper by Monica Pozzo, Chris Davies, David Gubbins and Dario Alfe framed the whole planet as a heat engine, with the growing inner core supplying both latent heat and chemical buoyancy to keep the outer core’s dynamo running.

• 5,400C is roughly the temperature at the inner core boundary, in the same range as the Sun’s visible surface
• 365 gigapascals is close to the peak pressure there, more than 3 million times sea-level atmospheric pressure
• 1 millimeter a year is the rough pace the inner core’s radius is thought to grow

But the physics has never quite added up on its own. Pure molten iron would need to cool 800 to 1,000 degrees below its freezing point before it would spontaneously crystallize, a process called supercooling, and geophysical evidence says the real core never got anywhere near that cold. A 2025 study from the University of Leeds proposed carbon as the missing piece, finding that carbon lets the core nucleate with only about 250 degrees Celsius of supercooling, far less than pure iron would demand. \u201cWe’re getting a rare glimpse into the chemistry of a region we can never hope to reach directly,\u201d said Alfred Wilson, the study’s lead author at the University of Leeds’ School of Earth and Environment.

How Old Is Earth’s Inner Core?

Nobody knows for certain. Estimates run from about 565 million years old to well over a billion, and the gap matters because the inner core’s growth is a major power source for Earth’s magnetic field. A younger core means the field ran on different fuel for most of the planet’s history.

There’s this huge range of 2 billion years where scientists think the inner core could’ve formed. These are the first field-strength data from the younger part of the range of possibilities suggesting that the inner core is really young.

John Tarduno, professor and chair of earth and environmental sciences at the University of Rochester, made that case in a study placing the core’s age at 565 million years, based on magnetic field strength preserved in ancient crystals.

• John Tarduno, University of Rochester reads ancient crystals as evidence the core is young, roughly 565 million years old
• Daniel Frost and colleagues, UC Berkeley modeled asymmetric core growth pointing to an age between 0.5 and 1.5 billion years
• Zuzana Konopkova and colleagues measured lower thermal conductivity in solid iron, which pushes the upper bound as high as 4.2 billion years

Berkeley seismologists reached their estimate a different way, building a growth model to explain why iron crystals in the core line up more along Earth’s rotation axis on one side than the other. \u201cDebate about the age of the inner core has been going on for a long time,\u201d said Daniel Frost, an assistant project scientist at UC Berkeley’s seismology lab, describing the faster growth detected under Indonesia’s Banda Sea.

A Sphere Hiding Inside the Sphere

The picture gets stranger closer to the center. Seismologists studying how earthquake waves cross the planet have described a distinct zone at the very middle of the inner core, roughly 650 kilometers across, where iron crystals appear arranged differently than in the shell surrounding it.

The shift shows up in P-waves, the fastest seismic waves, which change speed unexpectedly as they cross the planet’s core. Researchers behind the finding suggested it points to a major shift in cooling and crystallization, sometimes described as a \u201ccatastrophe,\u201d in Earth’s deep past, billions of years before now. The claim is still being tested against more earthquake data.

The Core Also Spins, Slows and Changes Shape

Rotation is not settled either. Earlier studies found the inner core once spun faster than the rest of the planet, then slowed enough around 2010 to appear to move backward relative to the surface, with the pattern repeating on what USC researchers have described as roughly a 70-year cycle.

A newer analysis of the same decades of earthquake doublets found something else going on at the same time. Analyzing seismic waveforms gathered from 1991 to 2024, researchers documented shape changes at the inner core’s shallow boundary, not just changes in spin. \u201cWhat we’re observing in this study for the first time is likely the outer core disturbing the inner core,\u201d said John Vidale, Dean’s Professor of Earth Sciences at the University of Southern California and the study’s lead author.

Even the Boundary Above It Is Rougher Than Expected

The boundary between the mantle and the outer core is not clean either. A high-resolution seismic survey beneath Antarctica found a thin, dense layer that may wrap much of the core, likely ancient ocean floor buried for hundreds of millions of years, with mountains up to five times taller than Everest rising from the core’s surface. \u201cWe are finding that this structure is vastly more complicated than once thought,\u201d said Samantha Hansen, a geologist at the University of Alabama who worked on the survey.

Separately, researchers at the University of Edinburgh reported in June 2026 that a broad region of iron-rich fluid beneath the equatorial Pacific, inside the outer core itself, switched from a weak westward drift to a stronger eastward one around 2010, based on ground and satellite data spanning 1997 to 2025.

None of this changes anything on the surface tomorrow. The inner core will keep freezing for billions of years yet, long after anyone alive today, adding its slow mill{“article”:”

Earth’s inner core is a solid ball of iron-rich metal about 1,220 kilometers in radius, smaller than the Moon but, by most estimates, heavier than it. Nobody has ever drilled to it or hauled back a sample. Everything geophysicists know about it comes from earthquake waves ringing through the planet like a struck bell.

For most of the last century, that was treated as a quiet, settled fact: a heavy iron marble sitting still at the planet’s center. The last eighteen months undid that picture. Seismologists, mineral physicists and laser physicists have independently reported that the core spins unevenly, changes shape, hides an extra layer within itself, and is built from a recipe still being argued over.

A Solid Iron Ball Smaller Than the Moon, Probably Heavier

The standard reference point for Earth’s interior is the Preliminary Reference Earth Model, published in Physics of the Earth and Planetary Interiors in 1981. It places the inner core’s outer edge about 5,150 kilometers below the surface, extending down to the planet’s center in a sphere with a radius of roughly 1,220 kilometers.

That is smaller than the Moon, whose mean radius runs about 1,737 kilometers. The comparison flips once mass enters the picture. Compressed by pressures far beyond anything at the surface, the inner core is estimated to weigh around 9 times 10 to the 22 kilograms, against the Moon’s 7.35 times 10 to the 22 kilograms.

Inge Lehmann, a Danish seismologist, first identified the boundary between the liquid outer core and a distinct solid center in 1936, working from seismograms recorded after a New Zealand earthquake. Later work by Beno Gutenberg, Charles Richter and a 1952 study by Francis Birch built the case that the sphere was crystalline iron. Japan’s 108 minutes to roll across the Moon’s surface took a palm-sized rover a matter of hours to arrange; nothing built has ever touched Earth’s own core, a few thousand kilometers under our feet.

How Frozen Iron Builds a Planet From the Inside Out

The inner core is not a fixed relic. It is growing. As Earth loses heat through the mantle and surface, the liquid outer core cools enough at the boundary for iron-rich metal to crystallize, adding fresh solid metal to the sphere’s surface at a pace of roughly one millimeter a year.

That figure is an order-of-magnitude estimate, not a measured annual tick. It depends on the heat flowing out of the core, the melting curve of iron alloys, and the composition of the surrounding liquid. Spread across a sphere more than 2,400 kilometers across, even a millimeter a year adds up to an enormous volume of metal over geological time.

Nobody has a confirmed recipe for what is actually freezing. Seismic data show the core is about 10 percent less dense than pure iron would be at the same pressure, meaning lighter elements are mixed in. A 2014 study in the Proceedings of the National Academy of Sciences, led by geophysicist James Badro, modeled the core as iron alloyed with nickel plus one or more lighter elements. The leading candidates:

• Oxygen – abundant in the mantle and plausible under core pressures
• Silicon – long favored by meteorite chemistry comparisons
• Sulfur – lowers iron’s melting point at high pressure
• Carbon – the focus of a 2025 Oxford study on how the core first froze

That last one turned out to matter. Researchers at the University of Oxford reported in Nature Communications that carbon, making up an estimated 2.4 to 3.8 percent of the core’s mass, could explain how the inner core managed to freeze at all. Pure molten iron needs to supercool by 800 to 1,000 degrees Celsius before it freezes, a threshold the core’s slow cooling history could never plausibly clear on its own. Mixing in carbon lowers that bar enough to make freezing plausible, resolving what researchers call the inner core nucleation paradox.

The inside is not uniform either. A study funded by the National Science Foundation, which repurposed seismic data originally gathered to monitor nuclear tests, confirmed inhomogeneity is everywhere inside the inner core, said Guanning Pang, the Cornell University researcher who led the work with University of Utah seismologist Keith Koper. Pang described the structure as a patchwork of different iron fabrics rather than a single uniform crystal.

Why 2025 and 2026 Became the Core’s Busiest Years

Zoom out and the pattern is hard to miss. In roughly eighteen months, independent teams working on three continents published findings that rewrote pieces of the standard picture.

1. February 10, 2025: USC seismologist John Vidale and colleagues report in Nature Geoscience that the inner core’s surface is changing shape, not just its rotation, based on decades of South Sandwich Islands earthquake doublets.
2. September 4, 2025: A University of Oxford team publishes the carbon nucleation study in Nature Communications, proposing how the core managed to freeze in the first place.
3. February 5, 2026: University of Liverpool researchers report in Nature Geoscience that two continent-sized hot rock structures at the base of the mantle, one beneath Africa and one beneath the Pacific, have shaped the outer core’s flow for millions of years.
4. June 1, 2026: Seismic analysis identifies a distinct zone roughly 650 kilometers wide at the very center of the inner core, an innermost inner core with its own crystal alignment.
5. June 11, 2026: University of Edinburgh researchers report that a broad current of molten iron beneath the equatorial Pacific reversed direction around 2010, based on satellite and ground data spanning 1997 to 2025.
6. July 7, 2026: Lawrence Livermore National Laboratory researchers use the National Ignition Facility to measure iron’s dynamic strength at core pressures for the first time, publishing in Nature Communications.

Six separate results, six separate teams, and not one of them was chasing a tidy headline. Together they describe a core that spins unevenly, deforms at its edges, hides a layer within itself, and helps steer currents thousands of kilometers above it.

Is the Inner Core Spinning Backward?

Yes, largely. Seismic evidence shows the inner core spun faster than the rest of Earth for decades, then slowed sharply around 2009 and 2010, appearing to reverse relative to the surface. Most researchers now think a rotation shift and a change in the core’s physical shape are both happening at once.

The evidence comes from earthquake doublets, pairs of quakes that strike the same spot in the South Sandwich Islands at different times, sending near-identical waves through the planet toward seismic arrays in Alaska and Canada. When the waveforms stop matching, something at the core has moved.

“The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbor the inner core on a human timescale,” said John Vidale, a geophysicist at the University of Southern California who led the 2025 Nature Geoscience study. “What we’re observing in this study for the first time is likely the outer core disturbing the inner core.”

Xiaodong Song, a Peking University seismologist who first reported the rotation anomaly, has argued the rotation and shape-change explanations are not competing. “It’s not either or,” he has said. Researchers at the Australian National University, who study the core independently, describe its outer surface as rough and mushy in places, deformed by internal stress, all while embedded in the liquid outer core and tugged by gravity from the mantle above.

Whether any of it reaches the surface is still unclear. “We don’t know that this is going to affect anything on the surface,” Vidale has said, “but we can’t say for sure until we figure out what’s happening.”

Nobody Agrees How Old the Inner Core Is

Solidity is one question. Age is a messier one. Estimates for when the inner core began freezing range from about 500 million years to more than 1.5 billion years, and the gap matters because it changes the story of how Earth’s magnetic field survived before the core existed at all.

• Paleomagnetic researchers, including Richard Bono, point to unusually weak and erratic magnetic field readings preserved in rocks roughly 565 million years old as evidence the inner core was just beginning to form around then.
• UC Berkeley seismologists, led by Daniel Frost, built a growth model showing lopsided crystallization, faster on one side of the core than the other, that only fits if the inner core is between 500 million and 1.5 billion years old.
• Thermal-history modelers counter that the field’s early strength, dating back as far as 4.2 billion years, is hard to explain if the solid core is truly that young, leaving core cooling rates as the biggest unresolved variable.

“Debate about the age of the inner core has been going on for a long time,” said Frost, an assistant project scientist at the Berkeley Seismological Laboratory. “The complication is: if the inner core has been able to exist only for 1.5 billion years, based on what we know about how it loses heat and how hot it is, then where did the older magnetic field come from?”

That lopsided growth pattern, tied to iron freezing out from molten iron more than half a billion years ago, remains one of the more widely cited data points on the young side of the debate. The same problem runs in the other direction too. Long-range models of Earth’s future, including one estimating that plant life could persist for another 1.87 billion years, lean on the same kind of compounding assumptions about heat and slow planetary change that make the core’s age so hard to pin down.

Labs Are Rebuilding the Core’s Furnace to Solve It

With no way to visit the core directly, researchers are increasingly building it in the lab instead. Three separate approaches are now running in parallel.

At Lawrence Livermore National Laboratory, physicists aimed the National Ignition Facility’s beams, ordinarily used to spark fusion reactions, at iron samples instead, pushing them to roughly 3 million atmospheres of pressure and 5,000 degrees Celsius. “We study iron because it is a primary constituent of Earth’s and other terrestrial planet cores, and how it functions under inner-core conditions is not well understood,” said Yong-Jae Kim, the LLNL physicist who co-led the study.

A separate Livermore team fired lasers at iron and found it kept forming the same hexagonal crystal structure under core-like pressure, according to research scientist Richard Kraus. A third group, pairing University of Texas at Austin mineral physicist Jung-Fu Lin with collaborators in China, fires projectiles into iron at ten times rifle-bullet speed to recreate similar conditions through shock waves.

None of the three methods can fully replace a seismic wave that has already crossed thousands of kilometers of real rock and metal. A review of core seismology in Nature Communications concluded future progress will likely come from tracking shear wave attenuation, anisotropy and tomography over longer stretches of time, the same basic tool Inge Lehmann used in 1936, pointed at a planet that turned out to be far less finished than it looked.

Frequently Asked Questions

How Do Scientists Study Earth’s Inner Core Without Ever Reaching It?

They rely almost entirely on seismic waves from earthquakes, which speed up, slow down or vanish depending on what they pass through far below the surface. The deepest borehole ever drilled reached just over 12 kilometers, a fraction of the roughly 5,150 kilometers separating the surface from the inner core’s edge, so seismology remains the only working tool.

What Is the Innermost Inner Core?

It is a separate zone roughly 650 kilometers wide sitting at the very center of the solid core, first identified through differences in how compressional seismic waves travel through it. Its iron crystals appear aligned east to west, while the surrounding outer layer’s crystals run north to south, a split that hints at a major shift in the planet’s cooling history billions of years ago.

Could Changes in the Inner Core Affect Life on the Surface?

Not in any way researchers have detected so far. Some patches of the inner core’s surface appear to rise and fall by as much as a kilometer within just a few years, changes one science journal compared to mountains rising and landslides tumbling, but those shifts occur thousands of kilometers down with no measurable effect at ground level yet.

Why Does the Inner Core Matter to Earth’s Magnetic Field?

Despite taking up less than 1 percent of Earth’s volume, the inner core is linked to about 10 percent of the total energy in the planet’s magnetic field, largely because its growth stirs the liquid metal around it. That stirring, not the solid core itself, is what actually generates the field.

Is the Inner Core Made of Pure Iron?

No. Meteorite chemistry, the traditional benchmark for planetary cores, points to iron and nickel plus maybe a few percent of silicon or sulfur, though scientists caution that comparison only gives a rough idea. Seismology and recent mineral physics experiments increasingly override meteorite estimates because they can be checked directly against how fast waves travel through the actual core.

How Fast Is the Inner Core Growing?

Roughly one millimeter a year on average, though growth is not even across the sphere. One widely cited model found the core grows faster beneath Indonesia’s Banda Sea than on the opposite side, with gravity redistributing the extra crystallized iron to keep the sphere roughly round.