It sounds like something from one of those cranks who believe the Earth is hollow, or a mash-up between Jules Verne's classics Journey to the Centre of the Earth and Twenty Thousand Leagues Under the Sea, but University of Alberta scientists claim to have evidence of a vast waterbody 500km beneath our feet.
The evidence for such a large claim is rather small – a few micrograms of water trapped inside a single gem, but according to Dr Graham Pearson, “That particular zone in the Earth, the transition zone, might have as much water as all the world’s oceans put together.”
Ringwoodite is named for Dr Ted Ringwood, an expert in the transformations that occur in certain rocks at high pressures. He showed that olvine (Mg, Fe)2SiO4 could take on a different crystal structure under enormous pressure and predicted that at great depths beneath the Earth's surface this would occur. Natural ringwoodite was first found in meteorites, rather than escapees from the deep.
It is thought that Ringwoodite is the most common mineral at depths of 520-660km, known as the lower transition zone, and that its properties shape the mantle's flow at these depths. Laboratory experiments have shown it can contain up to 2.6% water, but with no previous terrestrial examples geologists could only speculate as to how much might be absorbed here.
Pearson's ringwoodite was found in river gravel in Mato Grosso Brazil. The area is rich in diamonds brought up from the depths in kimberlite, a volcanic substance with the deepest origins of any rock commonly found at the surface. When ringwoodite rises from the enormous pressures below 520km it changes form to first wadsleyite and then olivine, but if trapped inside a diamond it keeps its original structure.
Pearons was looking for something else but was offered a brown diamond, too dirty-looking to have commercial value and weighing a tenth of a gram. Within this small stone is buried a 0.04mm piece of ringwoodite noticed by Pearson's student John McNeil, leading to publication in Nature. "It's so small, this inclusion, it's extremely difficult to find, never mind work on," Pearson said, "so it was a bit of a piece of luck, this discovery, as are many scientific discoveries."
After years of study with X-Ray diffraction and infrared spectroscopy its status as Ringwoodite was confirmed and the water content tested, returning a result of 1.5% by weight.
There are competing theories about whether the Earth at these depths is rich in water, or mostly dry. While the presence of water in Pearson's stone indicates the presence of water where it formed, the find may not be sufficient to convert those who believe in a dry transition zone to the theory that water is common at these depths. Professor Norm Sleep of Stanford University told Nature that even if you find a nugget of gold in a riverbed, “It would be unwise to assume that all the gravel in the stream is gold nuggets,”
Nevertheless, Pearson feels vindicated. "One of the reasons the Earth is such a dynamic planet is the presence of some water in its interior," Pearson said. "Water changes everything about the way a planet works."