'Diamonds, brought to the Earth's surface in violent eruptions of deep volcanic rocks called kimberlites, provide a tantalising window into the deep Earth.
A research team led by Prof Graham Pearson of the University of Alberta, Canada, studied a diamond from a 100-million-year-old kimberlite found in Juina, Brazil, as part of a wider project.
They noticed that it contained a mineral, ringwoodite, that is only thought to form between 410km and 660km beneath the Earth's surface, showing just how deep some diamonds originate.
While ringwoodite has previously been found in meteorites, this is the first time a terrestrial ringwoodite has been seen. But more extraordinarily, the researchers found that the mineral contains about 1% water.
While this sounds like very little, because ringwoodite makes up almost all of this immense portion of the deep Earth, it adds up to a huge amount of deep water.
Dr Sally Gibson from the University of Cambridge, who was not involved in the work, commented: "Finding water in such large concentrations is a hugely significant development in our understanding of the ultimate origin of water now present at Earth's surface."
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