When we learn about Earth's core at school, it looks like a calm sphere. One that's smooth, unperturbed and as round as a ping-pong ball. It sits cosily at the bottom of several layers that encase one another like a set of nesting dolls.
Of course, we’ve long known that the real picture is a lot more complicated than that. Earth’s iron-rich core generates a magnetic field that’s so strong it projects outwards beyond the planet’s surface and into space. There it reacts with the Sun’s solar winds to reveal what we call the magnetosphere.
Our observations of this projection of Earth’s core tell us that it’s constantly moving and nothing like a glowing ping-pong ball.
In fact, recent studies of seismic waves travelling through Earth’s layers paint a picture of the core’s edges so dramatic that it rivals the topology of the surface.
Indeed, the core seems to have its own landscape, with mountains, valleys, landslides and perhaps even volcanoes changing the core’s surface as we map it.
Amazingly, studies of seismic waves have revealed giant structures projecting into the lower mantle that are significantly taller than Mount Everest (almost 9km or 5.5 miles).
These 'mountains' are taller than any of the more traditional – surface – peaks in the Solar System, including the gigantic 22km-high (14 miles) Olympus Mons, a large shield volcano on Mars.
The mountains soaring upwards from Earth’s core can reach staggering heights of 1,000km (620 miles). That's more than 100 times the height of Mount Everest.
As scientists discover the core is a far more tumultuous place than we ever imagined, they’re naturally starting to ask if it has always been like this.
That’s the key question at the heart of a handful of projects using seismic waves to chart the core’s changing surface.
But what’s more exciting is that for every mystery they appear to solve, another anomaly seems to surface in their findings.
Two large mountains under the ground
Earth has two particularly large sub-surface mountains, one hidden under Africa and another nestled deep below the Pacific.
Both are about 2,000km (1,250 miles) beneath the surface, making them five times further away from us than the astronauts aboard the International Space Station.
The mountain under the Pacific is colossal, spanning about 3,000km (1,800 miles) – almost as wide as the Moon. Together, these mountains are big enough to account for six per cent of Earth’s entire volume.
It’s an unfathomable amount, but to put it in some sort of perspective, consider Earth’s vast oceans: they cover enough of the surface for us to refer to it as the Blue Planet, and yet all of the Earth’s vast oceans constitute just 0.12 per cent of the planet.
The subsurface behemoths have been named Tuzo and Jason, after the geologists Tuzo Wilson and W Jason Morgan.
And while they’re often referred to as mountains, technically they’re large low-velocity provinces (LLVPs). In other words, they’re areas that slow seismic waves down.
The effect they have on seismic waves was how they were discovered – initially in the 1980s, with further insights after a 1994 earthquake in Bolivia.
“Nobody knew if [LLVPs] were only a temporary phenomenon, or if they’ve been sitting there for millions, perhaps even billions, of years,” says Prof Arwen Deuss, a seismologist at Utrecht University in the Netherlands.
Subduction and slab graveyards
The LLVPs are surrounded by regions known as ‘slab graveyards’. They’re the final resting places of ill-fated tectonic plates. Tectonic plates are the interlocking puzzle pieces that comprise Earth’s surface.
When two plates rub up against one another, it can trigger an earthquake. But one plate can also slide beneath the other – a process known as subduction.
A subducting plate can sink all the way down through the mantle, becoming the geological equivalent of a shipwreck on the mantle floor – close to where it meets Earth’s core.
Recent research into LLVPs is calling into question what we thought we knew about these regions, however.
Deuss led a team looking at how seismic waves lose energy – or are ‘damped’ – as they pass through LLVPs.
Contrary to what they expected, Deuss’s team found that the waves lost very little energy. Much more energy was lost when the waves passed through the neighbouring slab graveyards.
After careful analysis, Deuss’s team concluded that the difference was down to the size of mineral grains that form the plates and mountains.
The grains in the graveyard plates are small and tightly packed, meaning it’s harder for seismic waves to pass through.
“The fact that the LLVPs show very little damping, means that they must consist of much larger grains,” Deuss says.
This discovery has important consequences for our understanding of Earth’s mantle as a whole. Big grains take far longer to assemble than smaller ones, meaning the LLVPs must be old – perhaps half a billion years old.
The mantle is usually considered to be a region of ‘fast-flowing’ material that’s well-mixed and constantly churning with eddies and currents, like the oceans above.
And yet these new results are hinting that massive structures, such as Tuzo and Jason, can persist for hundreds of millions of years.
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Sinking plates, rising mountains
They could even be far older. There’s a persistent and entrancing idea that these continent-sized structures in Earth’s mantle are giant fragments of a planet that hit Earth in its earliest days. Astronomers think that the debris from this collision snowballed into the Moon.
The fact that the grain sizes in the LLVPs are different to those in the surrounding tectonic plate graveyard, only adds to the intrigue.
A new study from March 2025 seems to pour cold water on the Moon link, however. A team led by Dr James Panton at Cardiff University found that Tuzo and Jason have different compositions.
“We find the Pacific LLVP to be enriched in subducted oceanic crust, implying that Earth’s recent subduction history is driving this difference,” he says.
This is just one way in which scientists are starting to learn more about the way material from Earth’s surface sinks into the mantle.
In late 2024, seismologists announced that they’d discovered a chunk of what was once the sea floor hiding in the mantle under the Pacific close to Easter Island.
This slab of crustal material began sinking 250 million years ago, at a time when the first dinosaurs appeared and all our modern continents were joined together.
That discovery was quickly followed by another in November 2024. A team led by Thomas Schouten, from ETH Zurich Geological Institute in Switzerland, used a new type of seismographic imaging to spot numerous patches in the mantle that appear, at first glance, to be subducted tectonic slabs.
Except they’re all in the wrong places. You’d expect them to be beneath joins in tectonic plates, but some of them are found in places that, as far as we know, have never experienced tectonic activity, such as below the western Pacific Ocean.
“With the new high-resolution model, we can see such anomalies everywhere in Earth’s mantle. But we don’t know exactly what they are,” Schouten says.
Seismic waves move through these regions at the same speed as they move through subducted slabs, but that doesn’t necessarily mean that’s what they are.
Given their sheer abundance, one potential alternative explanation is that they’re regions containing ancient, mineral-rich material that’s been in the mantle since the start. As such, they could be important ‘eyewitnesses’ to the birth of this planet.
It just goes to show how little we know about the world beneath our feet, its continued ability to surprise us and its importance in understanding how we came to be here.
Ultra-low velocity zones
There may be other records of Earth’s past lurking in the lower mantle, close to the boundary with the planet’s core.
In August 2024, a team led by geologist Dr Michael Thorne at the University of Utah found evidence for an extensive network of so-called Ultra-Low Velocity Zones (ULVZs). Seismic waves rippling through these regions can slow down by up to 50 per cent.
We knew about these zones before, finding them near hotspots like the volcanic islands of Hawaii. Yet Thorne’s research shows that they’re everywhere, including under North Africa, East Asia and the Pacific Northwest.
“These are some of the most extreme features discovered on the planet,” Thorne says.
Previously, scientists had suspected that ULVZs could be fragments of the space rocks that regularly rained down on Earth in its youth.
But the fact that they appear to be so widespread suggests that they’re more modern and are perhaps being actively generated today.
The exact mechanism remains unclear, but one idea put forward by Thorne is that material around tectonic mountain ranges on the sea floor – known as mid-ocean ridges – is melting.
This material is then scattered through the mantle where it accumulates at the boundaries of the LLVPs.
A disruptive doughnut-shaped discovery
Drop down below the mantle and you reach Earth’s core. Split into the inner and outer core, the whole thing has a diameter just shy of 7,000km (approx 4,300 miles).
That makes Earth’s core marginally bigger than the planet Mars. It’s like having a planet within a planet and, indeed, the core has many otherworldly properties. For one thing, the inner core is almost as hot as the surface of the Sun.
In August 2024, Dr Xiaolong Ma and Prof Hrvoje Tkalčić, a seismologist and geophysicist, respectively, both at the Australian National University, discovered a large, doughnut-shaped region of the core parallel to Earth’s equator.
Seismic waves travel through the region approximately two per cent slower than in the rest of the core. The pair estimates that this area is just a few hundred kilometres thick.
“We think this region contains more lighter elements, such as silicon and oxygen, and may play a crucial role in the vast currents of liquid metal running through the core that generate Earth’s magnetic field,” says Tkalčić.
Earth’s inner core may even have changed shape within the last 20 years, according to a separate study led by Prof John Vidale, a seismologist at the University of Southern California. The inner core of Earth is solid and rotates independently from the molten outer core.
Vidale thinks he’s spotted small, 100m-long (almost 330ft) deformations where the top of the inner core meets the outer core.
“The molten outer core is widely known to be turbulent, but its turbulence had not been observed to disrupt its neighbour, the inner core, on a human timescale,” Vidale says. “What we’re observing – for the first time – in this study, is likely to be the outer core disturbing the inner core.”
According to Vidale, such disturbances could even lead to volcanoes bubbling up along this boundary, alongside giant landslides. Although the sheer distances involved make these tricky things to prove conclusively.
The beginning of Earth's end
These changes are, perhaps, the beginning of the end for Earth and its habitability. Inch by inch, the molten outer core is freezing onto the solid inner core.
When it solidifies completely in a few billion years’ time, Earth’s magnetic field will switch off leaving us unprotected from the ravages of the Sun and its solar wind.
This stream of charged particles will gradually peck away at the planet’s atmosphere as it once did to Mars, turning Earth into a barren, lifeless wasteland.
The core has always been a bellwether for the current state of Earth. The modern core could even still hold clues as to how quickly the planet formed in the first place. It all comes down to a rare form of helium known as helium-3.
Helium is so light that it easily escapes from Earth’s gravitational clutches and leaks into space. However, the core could have imprisoned a supply of helium-3 when it formed from the giant cloud of gas and dust that was the early Solar System.
The trouble is that the core is largely made of iron and scientists have traditionally thought that iron and helium don’t mix.
Now a new study by researchers at the University of Tokyo in Japan has called that idea into question.
If helium-3 and iron do indeed mix, working out how much helium-3 there is in Earth’s core could help settle a longstanding debate about how quickly the planet formed.
If, as is most widely believed, Earth took about 100 million years to form, then the core won’t contain much helium-3, as it had ample time to leak out. A much higher level of helium-3 would suggest that Earth formed considerably faster.
Slowly but surely, geologists and seismologists are piecing together the clues that will allow them to pull back the proverbial curtain and learn the hidden secrets of Earth’s inner layers.
To date, they’ve revealed a region more mysterious and confounding than they had imagined. So, for now, we have more questions than answers.
But the more we discover, the more we’ll understand about the history of Earth and how it is that we came to call it home.
About our experts
Prof Arwen Deuss is a seismologist and professor of structure and composition of Earth's deep interior at Utrecht University, in the Netherlands. She has been published in various scientific journals, including Nature Reviews Earth & Environment, Nature Geoscience and Geophysical Journal International.
Dr James Panton is a research associate of geodynamics at Cardiff University, in the UK. He is published in the likes of Journal of Geophysical Research: Solid Earth, Geochemistry, Geophysics, Geosystems and Scientific Reports.
Thomas Schouten is a PhD candidate in geodynamics from ETH Zurich Geological Institute, in Switzerland. His work has been published in Scientific Reports, Geological Society London Special Publications and American Journal of Science.
Dr Michael Thorne is an associate professor of geology and geophysics at the University of Utah, in the US. He is published in Geophysical Journal International, AGU Advances and The Seismic Record, to name a few scientific journals.
Prof Hrvoje Tkalčić is the head of geophysics at Australian National University, in Australia. He is a published author with The Earth's Inner Core Revealed by Observational Seismology and Earthquakes: Giants That Sometimes Wake Up. He is also published in scientific journals such as Geophysical Journal International.
Prof John Vidale is a seismologist professor at the University of Southern California, in the US. He has been published in the likes of Bulletin of the Seismological Society of America, Geophysics and Nature.
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