Researchers from the Northwestern University and the University of New Mexico discovered evidence of a hidden ocean, deep in the Earth's mantle, 400 miles beneath North America. The hidden reservoir, not in the familiar liquid form, is apparently locked in a blue crystalline mineral called ringwoodite.

Researchers said that it may contain three times the amount of water as exists in all the world's surface oceans.

The discovery suggests that water from the Earth's surface can reach such great depths by plate tectonics, eventually causing partial melting of the rocks found deep in the mantle.

"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 Dr. Steven Jacobsen, an associate professor, in a statement. "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."

Scientists have long claimed that water is locked in a rocky layer of the Earth's mantle. The layer, known as the "transition zone" is located between the lower mantle and upper mantle and is estimated to be at depths between 250 miles and 410 miles.

Northwestern researchers are the first to provide a direct evidence of the region that extends across most of the interior of the United States.

The finding will be published in the journal Science and will help other studies in finding the amount of water locked up in mantle rock.