Earth is a jewel of the solar system, painted blue by the vast oceans that hold the majority of our planet’s water—or so we thought. Most of Earth’s water, according to a new study, may actually be locked in a reservoir 400 miles underground. That Earth, for much of its history, has been covered by water isn’t much of surprise. But how so much water ended up on a planet that, at its birth, was boiling hot has puzzled scientists for decades. Previous theories had assumed that Earth had picked up its water when it collided violently with icy comets and asteroids. But an alternate hypothesis just received a boost courtesy of a new study published in Science : Earth’s water may have been here all along, oozing out gradually as rock deep in the crust was pressed by the intense heat and pressure below the surface.

The idea of water trapped in solid rock isn’t as unlikely as it sounds. Minerals can trap molecules of water in their metals and metal oxides like a sponge. If the pressure gets high enough, the trapped water can get squeezed out. Steven Jacobsen, the Northwestern University professor who led the study, found water in subterranean ringwoodite, a deep blue mineral chemically similar to peridot, a green mineral often used in jewelry. Until a sample turned up in 2008 in a diamond coughed up from a volcano, ringwoodite had only been found in meteorites . The ringwoodite came from the “transition zone” between the upper and lower mantle, about 400 miles below the Earth’s surface, and about 1.5% of its weight turned out to be water. If a lot of this water-heavy mineral existed underground, scientists reasoned, that might be enough to explain where Earth’s oceans came from.

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So Jacobsen and his team went looking for dampness lurking deep underground by monitoring the seismic waves generated by earthquakes. Because the velocity of these waves changes depending on what kind of material they’re passing through—like, for example, if a mineral is wet or dry—measuring that speed can give geologists a map of what’s below the Earth’s surface. Andy Coghlan, writing for NewScientist :

Sure enough, they found signs of wet ringwoodite in the transition zone 700 kilometres [400 miles] down, which divides the upper and lower regions of the mantle. At that depth, the pressures and temperatures are just right to squeeze the water out of the ringwoodite. “It’s rock with water along the boundaries between the grains, almost as if they’re sweating,” says Jacobsen.

If all the ringwoodite in the transition zone is as damp as the samples that Jacobsen and his team detected, that layer would hold three times as much water as all of the Earth’s oceans combined, reducing their share from 96.5% of all known water to a relatively paltry 24.8%. In other words, the ringwoodite discovery could quadruple the amount of water found on Earth. A blue planet, indeed.

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