For some time now scientists have been studying ringwoodite for the possibility it records the existence of water deep below the earth’s mantle – formed in the transition zone, itself spanning 410-660 km below the surface, that volume of space suggests “several times the water of Earth’s oceans” could be contained underground 😮 according to this Quanta article:

A couple hundred pebble-size diamonds, plucked from Brazilian mud, sit inside a safe at Northwestern University. To some, they might be worthless. “They’re battered,” said Steve Jacobsen, a mineralogist at Northwestern. “They look like they’ve been through a washing machine.” Many are dark or yellow, far from the pristine gems of jewelers’ dreams.

Yet, for researchers like Jacobsen, these fragments of crystalline carbon are every bit as precious — not for the diamond itself, but for what is locked inside: specks of minerals forged hundreds of kilometers underground, deep in Earth’s mantle.

These mineral flecks — some too small to see even under a microscope — offer a peek into Earth’s otherwise unreachable interior. In 2014, researchers glimpsed something embedded in these minerals that, if not for its deep origins, would’ve been unremarkable: water.

Not actual drops of water, or even molecules of H₂0, but its ingredients, atoms of hydrogen and oxygen embedded in the crystal structure of the mineral itself. This hydrous mineral isn’t wet. But when it melts, out spills water. The discovery was the first direct proof that water-rich minerals exist this deep, between 410 and 660 kilometers down, in a region called the transition zone, sandwiched between the upper and lower mantles.

Read more here.