'Writing in the journal Nature, scientists on Wednesday said they had found an elusive mineral pointing to the existence of a vast reservoir deep in Earth's mantle, 400-600 kilometres (250-375 miles) beneath our feet.
It may hold as much water as all the planet's oceans combined, they believe.
The evidence comes from a water-loving mineral called ringwoodite that came from the so-called transition zone sandwiched between the upper and lower layers of Earth's mantle, they said.
Analysis shows that a whopping 1.5 percent of the rock comprises molecules of water.
The find backs once-contested theories that the transition zone, or at least significant parts of it, is 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."'
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