Researchers from Northwestern University and University of New Mexico have found deep pockets of magma located beneath North America, a likely signature of the presence of water at these depths.
This water is not in a form familiar to us - it is not liquid, ice or vapour.
This fourth form is water trapped inside the molecular structure of the minerals in the mantle rock.
The discovery suggests water from the earth's surface can be driven to such great depths by plate tectonics, eventually causing partial melting of the rocks found deep in the mantle.
"We are finally seeing evidence for a whole-earth water cycle that 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," explained geophysicist Steve Jacobsen from Northwestern University.
Scientists have long speculated that water is trapped in a rocky layer of the earth's mantle located between the lower mantle and upper mantle.
Jacobsen and seismologist Brandon Schmandt from University of New Mexico provided the first direct evidence that there may be water in this area of the mantle, known as the "transition zone," on a regional scale.
The findings converged to produce evidence that melting may occur about 640 km deep in the earth.
"H2O stored in mantle rocks, such as those containing the mineral ringwoodite, likely is the key to the process," researchers said.
Discovered this year from inside a diamond brought up from a depth of 640 km by a volcano in Brazil, a tiny piece of blue-coloured ringwoodite contained a surprising amount of water bound in solid form in the mineral.
"Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 80 km," Schmandt said.
If just one percent of the weight of mantle rock located in the transition zone is H2O, that would be equivalent to nearly three times the amount of water in our oceans.
The findings, published in the journal Science, will aid scientists in understanding how the earth formed, what its current composition and inner workings are and how much water is trapped in mantle rock.