The discovery of an elusive mineral, named after an Australian geologist, has led scientists to surmise there is a vast reservoir of water deep in Earth's mantle - as visualised by Jules Verne.
Writing in the journal Nature, scientists said they had found ringwoodite, pointing to the existence of water deep in Earth's mantle, 400km to 600km beneath our feet.
Ringwoodite is named after Australian geologist Ted Ringwood, who theorised that a special mineral was bound to be created in the so-called transition zone sandwiched between the upper and lower layers of Earth's mantle because of the ultra-high pressures and temperatures there.
The find backs once-contested theories that the transition zone, or at least significant parts of it, was water-rich, the investigators said.
"This sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area," said Graham Pearson of Canada's University of Alberta, who led the research.
"That particular zone in the Earth, the transition zone, might have as much water as all the world's oceans put together."
A piece of ringwoodite has been a long-sought goal. It would resolve a long-running debate about whether the poorly understood transition zone is dry or water-rich.
But until now, it has only ever been found in meteorites. Geologists had simply been unable to delve deep enough to find any sample on Earth.
All this changed in 2008 when amateur gem hunters digging in shallow river gravel in the Juina area of Mato Grasso, Brazil, found a tiny, grubby stone called a brown diamond.
Measuring just three millimetres across and commercially worthless, the stone was acquired by the scientists when they were on a quest for other minerals. But this turned out to be a bonanza.
In its interior, they found a microscopic trace of ringwoodite - the first terrestrial evidence of the ultra-rare rock.
"It's so small, this inclusion, it's extremely difficult to find, never mind work on," Pearson said in a press release, paying tribute to the diligent work of grad student John McNeill.
The team theorise the brown diamond rocketed to the surface during a volcanic eruption, hitchhiking in a stream of kimberlite, the deepest of all volcanic rocks.
Years of analysis, using spectroscopy and X-ray diffraction, were needed in specialised labs to confirm the find officially as ringwoodite.
Hans Keppler, a geologist at the University of Bayreuth in Germany, cautioned against extrapolating the size of the subterranean water find from a single sample of ringwoodite.
"In some ways it is an ocean in Earth's interior, as visualised by Jules Verne... although not in the form of liquid water," Keppler said in a commentary also published by Nature.