A groundbreaking series of studies has revealed what many geologists long suspected: Earth may hold vast quantities of water far beneath its surface—potentially three times the volume of all our oceans, stored not as liquid, but locked within the structure of deep mantle minerals.
The evidence comes from multiple fronts. In 2014, a team led by geophysicist Steve Jacobsen (Northwestern University) and seismologist Brandon Schmandt (University of New Mexico) published a landmark paper in Science showing seismic anomalies deep under North America that pointed to a hidden water cycle operating hundreds of kilometers down. But it was only in 2022 that the theory found direct mineralogical proof, thanks to a diamond unearthed in Botswana.
Now, the case for a “deep water Earth” has never been stronger—and the implications could reshape how we understand tectonics, volcanic activity, and even the long-term climate balance of our planet.
Water You Can’t Drink: How Ringwoodite Traps Earth’s Hidden Ocean
The heart of this discovery lies in a mineral called ringwoodite—a high-pressure form of olivine found in what geologists call the mantle transition zone, roughly 410 to 660 kilometers below the surface.
Under immense pressure and searing temperatures exceeding 2,000°F (1,100°C), ringwoodite acts like a sponge, absorbing water molecules not as vapor or liquid, but as hydroxyl ions embedded in its crystal structure.
Jacobsen’s lab work replicated these conditions using advanced high-pressure equipment and diamond anvil cells. “Ringwoodite can hold about 1% of its weight in water,” he told Northwestern Now in 2014. “That doesn’t sound like much, but when you consider the volume of rock down there, it adds up fast.”
His findings dovetailed with Schmandt’s seismic research using the USArray, a network of more than 2,000 sensors, which detected signs of partial melt—a telltale signature of water being released from minerals as they plunge deeper into the mantle. The only known process that could account for such melting at that depth was dehydration melting—when hydrated minerals like ringwoodite descend and release trapped water.
The Diamond That Cracked the Mystery Wide Open
In 2022, that indirect evidence became irrefutable. A paper in Nature Geoscience detailed the discovery of ringwoodite trapped inside a diamond recovered from the Karowe mine in Botswana. What set this gem apart wasn’t its sparkle—it was the presence of multiple hydrated mineral inclusions, confirming that water had been present where the diamond formed, around 660 kilometers down.
“The diamond acts as a geological time capsule,” explains study co-author Fabrizio Nestola. “It captures minerals from deep within Earth that would otherwise never reach the surface without decomposing.”
Even more striking, the inclusion wasn’t isolated. It showed a suite of hydrated minerals coexisting, suggesting a widespread, not localized, water-rich environment in the mantle transition zone.
Combined with prior data, the discovery confirmed that this part of the mantle likely holds a vast, planet-scale reservoir of water—not sloshing around like an underground lake, but intricately woven into the Earth’s mineral fabric.
A Deep Water Cycle Shaping Earth’s Surface
The idea of a “deep water cycle” is now gaining traction. Unlike the surface water cycle—evaporation, condensation, and precipitation—this hidden loop moves at geological timescales, driven by plate tectonics.
When oceanic plates subduct into the mantle at convergent boundaries, they carry water-rich sediments and rocks down with them. Some of this water gets stored in minerals like ringwoodite; some is eventually released again via volcanic activity, influencing surface processes such as eruptions and earthquake dynamics.
As Schmandt noted in 2014, “Geological processes on Earth’s surface are an expression of what’s going on inside the planet, out of our sight. We are finally seeing evidence for a whole-Earth water cycle.”
This reframes how scientists think about the origin and stability of surface water. Far from being a closed, surface-only system, Earth’s hydrosphere may be in constant exchange with the deep mantle—a buffering mechanism that could help explain the planet’s unique long-term climate stability.