Deep below our feet, far beyond the reach of drills or submarines, scientists think a hidden reservoir could reshape the story of Earth.
New lab experiments that mimic the brutal conditions at the planet’s centre suggest the core might be loaded with hydrogen. If that hydrogen ever combined with oxygen, it could represent the raw material for up to 45 Earth-sized oceans.
A buried clue to Earth’s missing water
For decades, geologists have argued over where Earth’s water came from. One camp backed the idea of icy comets and water-rich asteroids bombarding the young planet. Another argued that most of the water was here from the start, trapped in the rocks that built Earth and slowly released.
A new study, based on high-pressure experiments, gives a strong push to the second scenario. By recreating core-like conditions in the lab, researchers found that iron similar to Earth’s core can hold surprisingly large amounts of hydrogen.
Even if hydrogen makes up just 0.07–0.36% of the core by mass, it could correspond to the equivalent of 9 to 45 oceans of water.
That is not liquid water sloshing around next to molten iron. It is hydrogen atoms imprisoned in metallic alloys more than 2,900 kilometres below the surface. Yet it speaks volumes about how wet the early Earth may have been.
From seismic whispers to a complex core
The story of the core started a century ago, with seismology. By tracking how earthquake waves travelled through the planet, scientists realised Earth is layered. In 1936, Danish seismologist Inge Lehmann showed that a solid inner core sits inside a liquid outer core.
From those wave speeds, researchers could estimate density. The numbers suggested that the core is mostly iron and nickel. Meteorites made of metal, relics from the early Solar System, supported that view.
Yet something did not match. The core appeared too light to be pure iron-nickel. That meant other, lighter elements must be dissolved in it.
Light elements in a heavy heart
By the 1960s, scientists suspected that the core also contains lighter elements. Only in the last two decades have lab techniques become precise enough to realistically simulate core conditions: pressures above 100 gigapascals and temperatures of several thousand degrees Celsius.
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Today, most researchers agree that the core likely holds several light elements:
- sulphur
- silicon
- oxygen
- carbon
- hydrogen
How much of each remains uncertain. Hydrogen is especially tricky. It is the smallest, lightest atom and leaves a faint signature in measurements. All our knowledge comes indirectly, through simulations, lab experiments and seismic modelling.
Recreating the core with diamonds and lasers
To pin down hydrogen’s role, the research team turned to a specialised device called a diamond anvil cell. Two diamond tips squeeze tiny samples to enormous pressures, while lasers heat them to thousands of degrees.
The scientists pressed together two materials:
- an iron alloy resembling the composition of Earth’s core
- a hydrated silicate glass, representing the ancient magma ocean that once covered the young planet
The experiment ran at roughly 111 gigapascals and about 4,800 °C, conditions similar to the outer core. Under those extremes, elements can move between the molten silicate and the metal, just as they would have during Earth’s formation.
Once the samples cooled, the team scanned them in three dimensions at the nanometre scale using atom probe tomography. This high-resolution technique let them count individual atoms of silicon, oxygen and hydrogen within the metallic phase.
The measurements suggest that Earth’s core could store more hydrogen than many earlier models allowed, compressed into a metallic cage deep below the mantle.
| Parameter | Estimated value |
|---|---|
| Core hydrogen content (by mass) | 0.07–0.36% |
| Equivalent water volume | 9–45 modern oceans |
| Experimental pressure | ~111 GPa |
| Experimental temperature | ~4,800 °C |
What this says about the origin of Earth’s water
The location of hydrogen matters. If most of Earth’s water had arrived late, carried by comets after the core formed, the hydrogen should be concentrated in the outer layers: crust, oceans and atmosphere.
The new results suggest something else. Hydrogen appears able to partition into the metal that makes up the core, which implies that the building blocks of Earth already held substantial hydrogen when the planet was still molten.
Hydrogen locked in the core points to a “wet” origin for Earth, with water-bearing materials involved from the very start of planetary assembly.
That favours a scenario in which Earth formed from already hydrated rocks in the early Solar System, rather than being a dry ball later sprayed with ice. Comet impacts may still have contributed, but likely did not supply the bulk of our water.
Uncertainties and the need for more evidence
The authors of the study, published in Nature Communications, stress that their numbers remain provisional. Even tiny experimental biases can shift the hydrogen estimates significantly.
Core conditions vary with depth, and the early Earth went through violent phases of heating, mixing and giant impacts. Reproducing that full history in a lab is impossible. Other research teams will need to repeat and challenge these results with different techniques, compositions and pressure–temperature paths.
Sismology also has a say. As models of how seismic waves move through hydrogen-bearing alloys improve, scientists can test whether a hydrogen-rich core fits real earthquake data better than hydrogen-poor versions.
Why hydrogen in the core matters for life at the surface
Beyond the origin story of our oceans, hydrogen in the core could influence how the planet behaves today. The exact mix of light elements affects density, melting temperature and how easily the liquid outer core convects.
That convection drives the geodynamo, the process that generates Earth’s magnetic field. The field shields the atmosphere from charged particles from the Sun and helps prevent water loss to space. A subtle shift in the core’s recipe can ripple outward into long-term climate and habitability.
As hydrogen lowers the density of the core alloy, it may slightly alter how heat flows out of the deep interior. That, in turn, affects how the mantle circulates, how plates move and how volcanoes release gases, including water vapour and carbon dioxide.
Key concepts behind the science
Several technical terms sit at the heart of this research. A quick unpacking helps make sense of the claims.
- Diamond anvil cell: a device that squeezes tiny samples between two diamonds to reach pressures similar to those in planetary interiors.
- Atom probe tomography: a method where atoms are gently removed from a sharp needle-shaped sample and detected one by one, building a 3D chemical map.
- Magma ocean: a stage early in Earth’s history when much of the planet’s outer layer was molten, letting metals sink to form the core.
- Partitioning: the way elements divide themselves between different materials, such as between molten rock and liquid metal.
Understanding how hydrogen partitions between metal and silicate under extreme conditions gives researchers a handle on how much could end up in the core versus the mantle and surface.
What this means for other worlds
This hydrogen-rich core scenario has implications beyond Earth. Planets like Venus and Mars likely experienced their own magma oceans and core formation episodes. If similar processes operate there, hydrogen storage in their cores could help explain why their surfaces look so different from ours today.
For rocky exoplanets orbiting distant stars, the way water gets locked into the interior or released to the surface could shape whether they stay dry, become ocean planets or develop conditions suitable for life. Future models of habitability will need to factor in not just surface oceans but these deep, hidden reservoirs of hydrogen locked away under crushing pressure.