According to the Institute of Geosciences at Goethe University in Frankfurt, the transition zone between the Earth's upper and lower mantle contains significant amounts of water underground.
A systematic review
In a study just presented in Nature (here it is), the researchers analyzed a rare diamond formed 660 kilometers below the Earth's surface, using techniques such as Raman spectroscopy and FTIR spectrometry.
The findings confirm a long-standing theory that by accompanying subducting tectonic plates, ocean water enters the transition zone underground. This means, in summary, that the water cycle of our planet also includes the interior of the Earth.
The transition zone
“Transition zone” is the name given to the boundary layer that separates the Earth's upper mantle from the lower mantle. It is found at depths between 410 and 660 kilometers.
The immense pressure of the transition zone, up to 23.000 bar, causes the mineral olivine, which makes up about 70% of the Earth's upper mantle, to alter its crystalline structure. At the upper limit of the transition zone, about 410 kilometers underground, it transforms into wadsleyite (denser); at 520 kilometers it transforms into ringwoodite (even denser).
Wadsleyite and ringwoodite can (unlike olivine which is found at shallower depths) store large amounts of water, to the point that the transition zone could theoretically absorb six times the amount of water present in our oceans. And now we have confirmation that he is doing it, we still don't know to what extent.
Oceans underground
The diamond studied in Frankfurt was formed, as mentioned, at a depth of 660 kilometers: right between the transition zone and the lower mantle, where ringwoodite is the predominant mineral.
And the ringwoodite components within the stone have a high water content. “With this study,” he says Frank Brenker, lead author, “we have shown that the transition zone is not a dry sponge, but contains significant amounts of water.”
“This brings us one step closer to Jules Verne's idea of an ocean within the Earth,” says the researcher.
With the difference that down there there would be an "ocean" of rock soaked in water, and the underground Nautilus would have some problems crossing it.
