Scientists have found a massive reservoir of water buried about 700 kilometers below Earth’s surface, and it could hold three times as much water as all the oceans combined. This hidden ocean is trapped inside deep mantle rock and cannot be directly accessed or seen.
The discovery suggests that a lot of Earth’s water may have come from inside the planet itself, not just from comets or asteroids crashing into the early Earth. It could also explain why ocean levels have stayed relatively stable over hundreds of millions of years, even with continents drifting and climates changing.
Learning how water behaves deep in the mantle helps scientists understand Earth’s geology and how the oceans we know today have lasted so long.
Water Trapped in Ringwoodite
The water is not free-flowing but bound within ringwoodite, a rare blue mineral stable under the extreme pressures of the mantle’s transition zone, which extends roughly from 410 to 660 kilometers deep. According to the study published in Science, the mineral absorbs water at the molecular level, creating a sponge-like effect that allows the mantle to store enormous volumes.
Steven Jacobsenof Northwestern University, who led the research, explained that the team used about 2,000 seismographs across the U.S. and studied waves from over 500 earthquakes. Seismic waves move slower through wet rock, so by mapping where the waves slowed down, the researchers could figure out where water is trapped. He described it like using earthquakes as a kind of planetary MRI, showing what’s happening deep inside Earth.
He also stated that this discovery is strong evidence that Earth’s water partly came from inside the planet, and this deep reservoir helps prevent the oceans from flooding the surface over time.
“We should be grateful for this deep reservoir,” said Jacobsen. “If it wasn’t there, it would be on the surface of the Earth, and mountain tops would be the only land poking out.”
Extending the Water Cycle Far Below the Surface
This discovery also changes how we think about the water cycle. Water from the oceans gets dragged down into the mantle at subduction zones, where minerals like ringwoodite absorb some of it. Over millions of years, some of that water comes back to the surface through volcanoes and mantle upwelling, keeping the oceans relatively stable.
Jacobsen explained that this cycle shows Earth has a water system far bigger than what we see in clouds, rivers, and seas. It also helps explain why liquid water has lasted for billions of years, supporting life while planets like Mars or the Moon lost theirs long ago.
What Scientists Want to Learn Next
Most current seismic evidence comes from beneath North America. Researchers now aim to examine other regions to determine whether water-rich ringwoodite is common worldwide or concentrated in specific zones. They also hope to refine estimates of how much water the transition zone holds and how quickly it cycles in and out.
These insights are crucial for models of Earth’s formation, plate tectonics, and ocean stability over deep time. Each recorded earthquake offers a window into the planet’s hidden interior, revealing a vast, ancient system that has quietly shaped life on Earth.
While humans cannot access this water, its existence helps explain why our oceans have endured for billions of years and why Earth continues to support a global ocean and thriving ecosystems.
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