A reservoir of water lying deep under the Earth's surface may contain triple the volume of every one the world's oceans and may even be the "wellspring" source of them, U.S. researchers say.

Although not in a liquid form most familiar to all of us -- it is instead bound within rocks deep in the Earth's mantle -- it likely represents the largest reservoir of water on Earth, scientists at the University of New Mexico and Northwestern University say.

Writing in the journal Science, they report finding pockets of melted magma 400 miles underneath the North American continent that are likely signatures that water exists at those depths.

Scientists have long questioned whether the mantle, the rocky, hot layer between the Earth's crust and its core, might contain water bound up and trapped within rare minerals.

The new discovery is evidence water can be transported from the surface of the Earth to great depth by plate tectonics, the movement of continents and plates over the Earth's surface.

"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," says geophysicist and study co-author Steve Jacobsen at Northwestern. "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."

Movement and partial melting of rocks in the mantle's transition zone -- a region between the mantle's lower and upper layers from around 250 to 400 miles deep -- could allow water to become tightly bound to the minerals there, the researchers said.

To test if the transition zone could be a possible deep water reservoir, the researchers used seismic waves recorded during earthquakes to analyze the structure of the mantle in the zone and to detect if melting is taking place as tectonics drives rocks ever deeper.

"If we are seeing this melting, then there has to be this water in the transition zone," University of New Mexico seismologist Brandon Schmandt says.

"Melting is just a mechanism of getting rid of the water," he says.

If the surface water the Earth possesses today came from such degassing of molten rock, the researchers say, that's in contrast to the theory held by some scientists that water came to a young Earth by way of large, icy comets.

It is of course the existence of liquid water on the surface of the Earth that makes our planet habitable and capable of supporting life, which is why its origin is of such interest to science.

The latest findings are strong evidence of a process where water has long been cycling between deep interior reservoirs and the surface through the action of plate tectonics, the researchers say.

"Scientists have been looking for this missing deep water for decades," Jacobsen says.

And they may just have found it.

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