Are there ‘oceans’ hiding inside Earth?

The BBC has an article about a surprising theory:

We know more about the surface of Mars than we do the mantle of the planet we live on. As little as nineteen miles below the surface– the distance between the center of London, England and Heathrow Airport– the continental crust turns into the Earth’s mantle, a layer some eighteen hundred miles) thick that surrounds the Earth’s molten outer core.
Underneath the Earth’s oceans, the crust can be as little as three miles thick. And yet this trifling distance might as well be light years, for we know very little about this vital component of our planet.
For example, is there more water down there than we thought? For decades, scientists have postulated that the Earth’s oceans were created by comets striking the planet’s surface. But now some, like Professor Steven Jacobsen of Northwestern University, think that the rocks in the Earth’s mantle might have had a part to play as well; specifically a magnesium-rich silicate called ringwoodite. “I’m trying to ask big questions of where the Earth’s water came from,” he says. “One of the reasons I study rocks is they allow us to peer back in time.” Jacobsen had tried to replicate in the lab the kind of ringwoodite formed hundreds of miles down, but was unable to unless he added water.
As Jacobsen explains, the chance discovery of a diamond containing a lump of ringwoodite that had been spewed out of a volcanic vent millions of years ago showed that the mineral held as much water as the examples he had reformed in the lab. Ringwoodite could hold ten times as much water as previously thought, meaning that there could be oceans of water still sitting in the mantle rocks beneath us.

Rico says science is always finding something new, and Simon Redfern has another BBC article about it:

Minerals preserved in diamond have revealed hints of the bright blue rocks that exist deep within the Earth. They also provide the first direct evidence that there may be as much water trapped in those rocks as there is in all the oceans. The diamond (photo), from central-west Brazil, contains minerals that formed as deep as six hundred kilometers down and that have significant amounts of water trapped within them.
Researchers have published their findings in the journal Nature. The study suggests water may be stored deep in the interiors of many rocky planets.
Diamonds, brought to the Earth's surface in violent eruptions of deep volcanic rocks called kimberlites, provide a tantalizing window into the deep Earth.
A research team, led by Professor Graham Pearson of the University of Alberta in Canada, studied a diamond from a hundred-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 four and six hundred kilometers 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 one percent 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 in England, 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 the Earth's surface."
The observation is the first physical evidence that water can be stored in the deep interiors of planets, and solves a twenty-five-year-old controversy about whether the deep Earth is dry, wet, or wet in patches.
Discussing his findings, Professor Pearson told BBC News: "The discovery highlights the unique value of natural diamonds in trapping and preserving fragments of the deep Earth. It's incredible to think that, as you hold this sample in your hand, the residual pressure at the interface between the diamond and the inclusion is twenty thousand atmospheres." Describing his diamond sample, he said: "It looks like it's been to hell and back, which it has."
Professor Joseph Smyth of the University of Colorado has spent many years studying ringwoodite and similar minerals synthesized in his laboratory. He said: "I think it's stunning! It implies that the interior may store several times the amount of water in the oceans. It tells us that hydrogen is an essential ingredient in the Earth and not added late from comets. The Brazilian diamond was sculpted by corrosive fluids on its way up to the surface. "This discovery implies that hydrogen may control the interior processes of the Earth, just as it controls the surface processes, and that water planets, like Earth, may be common in our galaxy."
A key question posed by the observation is to understand the extent to which plate tectonics on Earth leads to oceans of water being recycled deep within our planet, and to predict the likely amounts of water trapped in other rocky planets.
Ringwoodite is expected to form deep in Mars as well, for example, where it sits against the metallic core. Grains of the same mineral synthesized in Professor Smyth's laboratory shine bright blue under the microscope. Given the new findings of ringwoodite's water-bearing capabilities, its abundance at depth, and its beautiful hue, the term "blue planet" seems even more appropriate for Earth.