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It has been a longstanding mystery for scientists - how water arrived on Earth.

Now, researchers say they may have an answer - it was here all along.

A new theory claims that giant underwater 'oceans' of water is stored in porous rock. 

Researchers at Ohio State University believe the same amount of water that currently fills the Pacific Ocean could be buried deep inside the planet right now - and that underground 'oceans' or rock were created at the same time the earth was.

Researchers at Ohio State University believe the same amount of water that currently fills the Pacific Ocean could be buried deep inside the planet right now.

The new study is helping to answer a longstanding question that has recently moved to the forefront of earth science: Did our planet make its own water through geologic processes, or did water come to us via icy comets from the far reaches of the solar system?

The answer is likely 'both,' according to the team, who are to present their work at the American Geophysical Union (AGU) meeting in San Francisco tomorrow.

They found a previously unknown geochemical pathway by which the Earth can sequester water in its interior for billions of years and still release small amounts to the surface via plate tectonics, feeding our oceans from within.

'When we look into the origins of water on Earth, what we're really asking is, why are we so different than all the other planets?' Wendy Panero, associate professor of earth sciences at Ohio State said.

'In this solar system, Earth is unique because we have liquid water on the surface. 

'We're also the only planet with active plate tectonics. 

'Maybe this water in the mantle is key to plate tectonics, and that's part of what makes Earth habitable.'

Central to the study is the idea that rocks that appear dry to the human eye can actually contain water—in the form of hydrogen atoms trapped inside natural voids and crystal defects.

Oxygen is plentiful in minerals, so when a mineral contains some hydrogen, certain chemical reactions can free the hydrogen to bond with the oxygen and make water.

Stray atoms of hydrogen could make up only a tiny fraction of mantle rock, the researchers explained. 

Given that the mantle is more than 80 percent of the planet's total volume, however, those stray atoms add up to a lot of potential water.

This plate tectonics diagramshows how mantle circulation delivers new rock to the crust via mid-ocean ridges. New research suggests that mantle circulation also delivers water to the oceans.

In a lab at Ohio State, the researchers compress different minerals that are common to the mantle and subject them to high pressures and temperatures using a diamond anvil cell—a device that squeezes a tiny sample of material between two diamonds and heats it with a laser—to simulate conditions in the deep Earth. 

They examine how the minerals' crystal structures change as they are compressed, and use that information to gauge the minerals' relative capacities for storing hydrogen. 

Then, they extend their experimental results using computer calculations to uncover the geochemical processes that would enable these minerals to rise through the mantle to the surface—a necessary condition for water to escape into the oceans.

Where it is: Researchers say the minerals called ringwoodite and wadsleyite can store water like a sponge, and reveals the Earth's transition zone could be a vast reservoir of water

In a paper now submitted to a peer-reviewed academic journal, they reported their recent tests of the mineral bridgmanite, a high-pressure form of olivine. 

While bridgmanite is the most abundant mineral in the lower mantle, they found that it contains too little hydrogen to play an important role in Earth's water supply.

Another research group recently found that ringwoodite, another form of olivine, does contain enough hydrogen to make it a good candidate for deep-earth water storage. 

So Panero and Pigott focused their study on the depth where ringwoodite is found—a place 325-500 miles below the surface that researchers call the 'transition zone'—as the most likely region that can hold a planet's worth of water. 

Graham Pearson holds the first terrestrial sample of ringwoodite ever found - which led to the discovery

From there, the same convection of mantle rock that produces plate tectonics could carry the water to the surface. 

For the research presented at AGU, Panero and Pigott performed new computer calculations of the geochemistry in the lowest portion of the mantle, some 500 miles deep and more. 

There, another mineral, garnet, emerged as a likely water-carrier—a go-between that could deliver some of the water from ringwoodite down into the otherwise dry lower mantle.

If this scenario is accurate, the Earth may today hold half as much water in its depths as is currently flowing in oceans on the surface, Panero said—an amount that would approximately equal the volume of the Pacific Ocean. 

This water is continuously cycled through the transition zone as a result of plate tectonics. 

'If all of the Earth's water is on the surface, that gives us one interpretation of the water cycle, where we can think of water cycling from oceans into the atmosphere and into the groundwater over millions of years,' she said. 

Schematic partial cross section of the Earth showing the location of ringwoodite, which make up approximately 60% by volume of this part of the transition zone. The diamond containing the water-bearing ringwoodite inclusion found by originated from approximately 500 km beneath the Earth's surface, where a large mass of water may accumulate by the subduction and recycling of oceanic lithosphere, into the transition zone.

'But if mantle circulation is also part of the water cycle, the total cycle time for our planet's water has to be billions of years.'

Earlier this year Canadian researchers say analysis of a rare mineral points to the huge store of water deep in Earth's mantle, 400-600 kilometres (250-375 miles) beneath our feet.

It echoes the hundred and fifty year old novel, 'Journey to the Centre of the Earth', in which French science-fiction forerunner Jules Verne pictured a vast sea that lay deep under our planet's surface.

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 in the journal Nature.

Analysis shows that 1.5% 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.' 

The $20 diamond that led to the discovery: Diamond sample JUc29, from Juina, Brazil, containing the hydrous ringwoodite inclusion. The rough diamond has been naturally sculptured to its unusual shape by corrosive mantle fluids during transport to the surface.

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.

And he also said the water was likely to be locked up in specific rocks, in a molecular form called hydroxyl.

'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.

The implications of the discovery are profound, Pearson suggested.

If water exists in huge volumes beneath Earth's crust, it is bound to have a big impact on the mechanics of volcanoes and the movement of tectonic plates.

'One of the reasons the Earth is such a dynamic planet is the presence of some water in its interior. Water changes everything about the way a planet works,' said Pearson.