A battered diamond found in Juina, Mato Grosso, Brazil that came from 500km deep and formed at about 200,000 atmospheres of pressure (gem quality diamonds come from about 130-200km deep) has been shown to contain a microscopic speck of ringwoodite. Ringwoodite had been suspected to exist there but now we have proof. Although it is only hydrated by 1.5% water this suggests that there is as much water bound in that region of the interior at 410-660km deep as in the whole of our oceans.
"No-one is ever going to run a geological field trip to the transition zone 500km beneath the Earth's surface, and no-one is ever going to drill down to the transition zone," said Pearson [Canada Excellence Research Chair at the University of Alberta and author with et al of the Nature paper]. "It was a total piece of luck that we found this."