The surprising ocean hidden inside Earth’s mantle

Researchers from Northwestern University and the University of New Mexico have uncovered groundbreaking evidence of potentially massive amounts of water trapped deep beneath the United States — not as liquid oceans or flowing rivers, but locked inside the very rocks of the Earth’s mantle.

While we all know Earth as the “Blue Planet” thanks to its abundant surface water, scientists have long wondered how much water is actually cycling deep within the planet, far below where we live. This new discovery suggests that water is stored deep in a mysterious region of the Earth’s interior called the transition zone, located roughly 250 to 410 miles beneath North America.

What did they find?

Using a combination of seismic data from a vast network of over 2,000 seismometers (the USArray) and high-pressure laboratory experiments, the researchers found pockets of partially melted rock, or magma, about 400 miles underground. This melting appears to be caused by water stored within a mineral called ringwoodite — a rare form of rock that can trap water within its crystal structure under extreme pressures and temperatures.

“If just one percent of the weight of the mantle rock in the transition zone is water, that would be nearly three times the volume of all Earth’s oceans combined,” says Northwestern geophysicist Steve Jacobsen, co-author of the study.

How is water stored so deep?

The water isn’t liquid like the water we swim in. Instead, it’s chemically bound inside minerals under crushing pressure and heat exceeding 2,000°F. At these depths, water molecules split and attach as hydroxyl groups (OH) inside the mineral structures, effectively creating a “hidden ocean” trapped inside solid rock.

This discovery builds on a 2014 finding of a tiny piece of ringwoodite inside a diamond brought to the surface by a volcanic eruption in Brazil — the only direct sample we have from this part of the mantle — which contained surprising amounts of water.

Why does this matter?

Understanding how much water is trapped deep inside the Earth is key to solving many geological mysteries. Water in the mantle affects melting, volcanic activity, and plate tectonics — the processes that shape earthquakes, mountain building, and even the magnetic field that protects our planet.

“The geological activity we see on the surface — from volcanoes to earthquakes — are expressions of these deep processes,” says co-author and seismologist Brandon Schmandt from the University of New Mexico.

The study offers the first direct evidence for a whole-Earth water cycle, where water is transported from the surface deep into the mantle and then possibly cycled back again, influencing Earth’s interior dynamics and surface habitability.

How did they study this hidden water?

Jacobsen simulated the extreme conditions of the mantle in his lab using diamonds as anvils to compress minerals and studied their behavior with intense X-rays and infrared light. Meanwhile, Schmandt analyzed seismic waves traveling through the Earth, which slow down when passing through partially melted regions, revealing “signatures” of water-induced melting deep below North America.

This melting, called dehydration melting, happens because minerals in the transition zone can hold water, but once they move deeper into the lower mantle, they must release it — causing partial melting at the boundary.

Conclusion

This discovery marks a monumental step forward in our understanding of Earth’s inner life and its water cycle—a cycle that reaches far beyond the surface into the planet’s very mantle. The notion that vast “hidden oceans” exist locked within the rocks beneath us not only reshapes how we think about Earth’s water reservoirs but also sheds light on the dynamic processes that sustain life on the surface. As scientists continue to unravel these deep mysteries, we edge closer to answering fundamental questions about Earth’s formation, its ongoing evolution, and what makes it a uniquely vibrant and habitable world. Truly, the story of our planet is one of connection—between the hidden depths beneath our feet and the waters that cradle life all around us.