The journal Nature announced a diamond discovery that helped settle a dispute about the Earth’s composition. The dispute concerned whether there is water in the transition zone — the portion of the Earth’s mantle 250 to 410 miles underground. Skeptics have argued that water cannot exist at that depth...At around 310 miles underground, wadsleyite is pressed into ringwoodite. Both substances can hold water. Now, geophysicists working in Brazil have uncovered a diamond containing ringwoodite that came steaming out of a volcano...The paper emphasized the volume of water likely to be present in the transition zone — possibly more than all of the oceans combined. But it’s at least 250 miles underground, and we’ve never drilled deeper than seven.
The subject is covered in greater detail at Scientific American, where they indicate that the water in these minerals is carried to the mantle via plate tectonics:
Plate tectonics recycles Earth's crust by pushing and pulling slabs of oceanic crust into subduction zones, where it sinks into the mantle. This crust, soaked by the ocean, ferries water into the mantle. Many of these slabs end up stuck in the mantle transition zone. "We think that a significant portion of the water in the mantle transition zone is from the emplacement of these slabs," Pearson said. "The transition zone seems to be a graveyard of subducted slabs."
The Nature article is here.