Science teachers may have to add a whole new layer to the water cycle.

Scientists have discovered evidence of a vast reservoir of water hiding up to 400 miles beneath the surface.

The discovery could transform our understanding of how the planet was formed, suggesting that Earth's water may have come from within, rather than from collisions with large, icy comets.

The water is trapped in a blue mineral called ringwoodite that sits in the mantle, a hot, rocky layer between the Earth's crust and outer core. That means the water is not the familiar liquid, vapor or ice, but a fourth, mineral form. We reported earlier this year on a rare diamond containing a microscopic piece of ringwoodite that bolstered evidence for the vast wet zone.

It is likely the largest reservoir of water on the planet, and could be the source of the oceans' liquid. The study was published in the journal Science.

The study is also remarkable for the discovery that melting and movement of rock occurs in a layer of the mantle known as the transition zone, between the upper and lower mantles, the Guardian reports. Most melting was thought to occur at much shallower depths.

"Geological processes on the Earth's surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight," said Geophysicist Steve Jacobsen from Northwestern University, co-author of the study.

"I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades," he said.

The study relied on seismometers across the U.S. and lab experiments simulating rocks under high pressure, says Nature World News.

"Ringwoodite here is key," it notes. "Its crystal-like structure makes it act like a sponge and draw in hydrogen and trap water."

It could be a vast amount of water, says the Guardian. "If just 1 percent of the weight of mantle rock located in the transition zone was water it would be equivalent to nearly three times the amount of water in our oceans, Jacobsen said."

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