Tree-covered hills stand watch over a yawning hole in the ground. The giant pit steps down in stages, its concentric rings growing ever smaller the deeper it goes. Huge trucks rumble up and down these makeshift roadways.
This is the scene at Wangu goldfield in China’s Hunan province. This area has already been mined for gold, but it seems they’ve only scratched the surface.
In late 2025 came the announcement that they believe there’s over one thousand tonnes of gold tucked away below the existing mine.
If confirmed, that would make it the biggest deposit of gold – and perhaps the largest deposit of any precious metal – ever found. That amount of gold has a market value of around £63 billion (approx. $83 billion).
So you’d be forgiven for thinking that gold is rare. After all, its apparent scarcity has made it a store of value in human culture for millennia.
But gold isn’t rare on Earth at all. It’s only rare at the surface. “99.9 per cent of Earth’s precious metals are locked in the core,” says Prof Matthias Willbold from the University of Göttingen in Germany.
For decades, most geologists believed that’s where they stayed. But Willbold is among a growing number of scientists pushing a relatively new idea: that the Earth’s core is leaking out.
This is still the subject of frenzied debate, but working out who is right has big consequences. Not just for our understanding of Earth, but for how common planets like ours may be in the wider Universe.
Down to the core
1. Crust
The crust is split into the oceanic crust, which is a maximum of 10km (6 miles) thick, and the continental crust, which can be as much as 80km (50 miles) thick in places.
The crust rises and falls by up to 25cm (10 inches) each day as the Moon pulls on it.
2. Mantle
Together, the crust and the top half of the mantle make up the lithosphere, which is broken into tectonic plates that shift. These shifts cause earthquakes and the continents to drift.
The mantle is by far the largest part of the Earth, making up 84 per cent of its total volume.
3. Outer Core
This is the only truly liquid layer of Earth’s internal structure. Around 2,000km (1,250 miles) thick, the outer core is mostly iron and nickel, with between five and ten per cent made up of lighter elements.
The transition between the inner core and outer core is located approximately 5,150km (3,200 miles) beneath Earth’s surface.
4. Inner Core
A solid, crystalised iron structure around 70 per cent the size of the Moon. It is around 5,000°C (9,032°F) – almost as hot as the surface of the Sun – but the intense pressure has forced it to solidify.
Its interactions with the outer core are responsible for Earth’s magnetic field.
In the beginning
Earth formed 4.54 billion years ago when detritus left over from the formation of the Sun coalesced under gravity. These planetesimals smashed together with such force that the Earth was initially completely molten.
The heaviest ingredients, such as iron and nickel, sank to the baby planet’s core.
But they didn’t go alone. “Iron-loving elements were dragged into the core,” says Willbold. These elements, known as ‘siderophiles’, bond readily with iron and so were chemically drawn into the core as it formed.
They include gold, tungsten, platinum and ruthenium.
Our view of this hidden interior comes not from drills or cameras, but from vibrations. When earthquakes strike, they send seismic waves rippling through the planet. Some slow down, others speed up. Some get reflected, others bend.
By measuring how these waves travel, geophysicists have mapped the planet’s layers as if performing a medical scan on a patient.
From these signals came the first picture of the core, showing two nested spheres, one liquid, one solid, separated from us by nearly 3,000km (1,865 miles).
If the siderophiles were dragged into this core long ago, how come we still find these metals at the surface? One option is that they are being transported up from the layer between the Earth’s core and crust – the mantle.
“There’s some geophysical evidence that plumes [mantle upwellings of hot rock] reach quite deep into the mantle and probably originate at the core-mantle boundary,” says Willbold.
These mantle plumes can reach the crust in volcanic places such as Hawaii and Iceland.
But the question remains: how did these elements end up in the mantle? That’s where geologists start to disagree.
Traditionally, they have looked beyond Earth for an answer. “We have to re-enrich the highly siderophile elements in the Earth’s mantle by some sort of process,” says Dr Mario Fischer-Gödde from the University of Cologne, Germany.
“The most famous explanation is that we bring it back with meteorites.”
Evidence from cratering on the Moon and other solid Solar System bodies points to a spike in impacts around 3.9 billion years ago. This period – known as the Late Heavy Bombardment – occurred when Earth was around 650 million years old.
During this time, Earth was pelted by huge asteroids made of the same ancient dust that built the Earth, meaning they were rich in heavy elements.
Some of the flecks of gold now mined in places like Wangu could be traces left by these impacts – a chemical echo of the planet’s earliest moments.
As Earth’s core had already separated out, the new metals couldn’t sink any further and so they became trapped in the mantle instead.
These asteroids overprinted the original chemistry of our planet, leading to the geological landscape we see today.
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A leaky core?
There is, however, a second theory as to why geologists are finding elements in our mantle that should be confined to the core: our core is leaking.
To examine the first clue to this theory, let’s start with tungsten. As a siderophile element, any tungsten the Earth had to start with sank to the core. But a rarer form of tungsten can also be produced by the radioactive decay of an element called hafnium.
Crucially, this tungsten-182 has two fewer neutrons than the more common tungsten-184.
As hafnium is a rock-loving element, it wasn’t dragged down to the core with the siderophiles and so remains in the mantle. As it’s decayed over time, it’s enriched the mantle with more tungsten-182 than what’s found in the core.
However, there are some pockets within the mantle that have a higher tungsten-184 to tungsten-182 ratio than average.
“If you add a tiny bit of core material into the mantle, then you would see that,” Willbold says. Perhaps some of the primitive core material – richer in the normal isotope tungsten-184 – is leaking out and boosting the ratio.
Tungsten isn’t the only clue. Another metal – ruthenium – tells a parallel story, but not everyone reads it the same way.
When researchers compare ruthenium isotopes in Earth’s mantle with those found in meteorites, they match. As meteorites are simply pieces of asteroids that fell to Earth, this would suggest that ruthenium, at least, was brought to Earth during the Late Heavy Bombardment.
But a few deep-sourced rocks originating from close to the core-mantle boundary, especially ones linked to mantle plumes such as those in Hawaii, show a faint but consistent deviation.
They have a slightly different mix of ruthenium isotopes, hinting that they may have mixed with something that doesn’t match any known meteorite.
For Willbold, that deviation in ruthenium might be a whisper from the core itself. And it’s an idea that could be tested.
“You would look for this anomalous ruthenium isotopic composition in rocks which probably come from the core-mantle boundary,” he says.
Fischer-Gödde is sceptical. “The isotopic signatures that we observe can be interpreted as tungsten and ruthenium mixing back into the mantle from the core, but it’s not definitive proof,” he says.
“We have to ask why it’s only these two elements.” In other words, why haven’t we found evidence of the other iron-loving elements also spilling out of the core?
When it comes to gold, it’s tricky because it only has one stable isotope – gold-197 – so geologists can’t compare isotope ratios like they did with tungsten and ruthenium and look for oddities. It should be possible for other siderophiles such as platinum, palladium, osmium and iridium.
Osmium, in particular, is well measured but the ratio is as expected. Simply, there are no hints that osmium is leaking out of the core.
Fischer-Gödde believes the tungsten and ruthenium anomalies can be explained through meteorites alone, without the presence of a leaky core. The Late Heavy Bombardment was not a singular event, but rather a drawn-out cosmic deluge.
The asteroid fragments that rained down on the early Earth likely came from far and wide across the Solar System, each forming in different conditions.
These distant cousins carried slightly different isotopic fingerprints, enough to leave a faint, uneven signature in the mantle to this day.
Full of hot air
Part of the problem is that the story of Earth’s metals is a messy one. Untangling whether they pre-date the planet or resulted from radioactive decay at a later date is almost impossible.
A potential way through this impasse is to pivot away from metals and towards gases. One gas in particular could play the role of arbitrator: helium.
Helium is the second most abundant element in the Universe after hydrogen, accounting for roughly a quarter of all atomic matter by mass. An ordinary atom of helium – helium-4 – has two protons and two neutrons.
Helium-3, which only has one neutron, is considerably rarer.
Despite its cosmic abundance, both forms of helium are rare on Earth because they are so light that they escape our gravity and drift into space.
The difference between them is that helium-4 is continually produced as a byproduct of radioactive decay, whereas helium-3 is not.
Any helium-3 still left on Earth must be primordial, trapped inside the planet when it formed.
When geologists sample volcanic plumes that reach the surface in places such as Hawaii and Iceland, they find that the helium-3 to helium-4 ratio can be up to 30 times higher than in the atmosphere.
In other words, a significant amount of primordial helium-3 is still making it to the surface from deep inside the planet.
The big question is: how deep? Could the helium-3 be leaking out of the core? That is still an open question, but answering it could be the key to unlocking the secrets of what’s going on in the heart of our planet.
If all this talk of a leaking core sounds ominous, you needn’t worry. “It’s not like there’s a river of metal flowing up,” Willbold says.
The quantities involved are vanishingly small – grams per year of metals, a few kilograms of gas at most. Over Earth’s lifetime, that adds up to almost nothing. The core isn’t draining away. It’s just breathing.
A perfect position
The effect may be small, but it still matters. “If the core and mantle have been exchanging even trace material for billions of years, that changes how we think about the planet’s evolution,” Willbold says.
Fischer-Gödde agrees: “You could imagine two identical planets, but if one seals up and the other leaks, they’ll have completely different geological lifetimes.”
To underscore this point, we only need to look at our neighbouring planets in the Solar System.
Mars, which is only about half of Earth’s diameter, cooled quickly. Its core solidified, causing its magnetic field to vanish. In turn the Sun pecked away at the Martian atmosphere until barely anything was left, meaning Mars’s liquid water disappeared too.
Venus has swung the other way. At almost the size of Earth, it has plenty of internal heat – but unlike our planet Venus doesn’t have tectonic plates. These allow some of the heat of our own planet to vent out, but this can’t happen on Venus.
Earth seems to sit in the sweet spot between the two. Its various layers are still interacting. Perhaps the secret to Earth’s enduring stability lies not only in the fact that its insides are still leaking, but they are leaking just the right amount.
Deep within that interaction zone sit two colossal structures, each larger than a continent, resting at the base of the mantle. Seismologists call them large low-shear-velocity provinces, but most just say ‘the blobs’.
One lies under Africa, the other beneath the Pacific.
They rise for thousands of kilometres and may be denser, hotter in patches where material from the core accumulates before feeding mantle plumes. If anything acts as a conduit between the core and crust, these provinces may be it.
Understanding Earth’s inner workings also has important implications for our search for life beyond our own Solar System.
“When we look at exoplanets, we’re basically trying to work out whether they’re still hot enough inside to do what Earth does,” says Willbold. “We can’t see their interiors, but we can model whether they’re still geologically alive.”
The next steps are already under way. Researchers are trying to measure the isotopic ratios of other siderophiles to see whether the same subtle offsets appear.
High-pressure experiments aim to reproduce core–mantle mixing in the lab. New seismic models may reveal whether the blobs truly connect to the core or are relics of ancient sunken tectonic plates.
Each clue helps to refine the picture of a planet with layers that aren’t sealed off from one another, but gently interacting.
Earth, it seems, has never truly stopped evolving. Far below even our deepest mines, a slow conversation continues between the world’s metallic heart and its rocky shell.
A dialogue that may be the secret to understanding how we came to call this planet home.
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