That’s quite interesting EG, ringwoodite where the water is.
"When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit," Schmandt said. "This is called dehydration melting.” After the rock melts, the researchers say, the water becomes trapped in the transition zone, creating a reservoir.
In March, a paper published in nature mag from a different research group used a series of techniques including x-ray diffraction and infrared spectroscopy to confirm that a ringwoodite sample (the first to ever come from within the Earth and not just created in a lab) had a water content above one percent. This quantity matches what has been predicted by Schmandt’s experiments. Earth’s mantle is so vast, that if 1% of the material in the transition zone is actually water, it would represent a reservoir three times larger than all of Earth’s oceans combined.
https://www.iflscience.com/environment/huge-underground-ocean-discovered-towards-earths-core/