In his classic 1864 adventure yarn "Journey to the Center of the Earth", Jules Verne tells the story of Professor Otto Lidenbrock and his companions, who descend into a volcanic crater in Iceland, and through a system og tunnels and caves, find their way to a huge cavern in the interior of the Earth, with an ocean and continents filled with all manner of ancient species of life.

And while Jules Verne might have been a bit off on the details, a seismologist at Washington University in St. Louis, working on the first 3-D model of seismic wave damping deep in the Earth's mantle, has revealed the existence of an underground water reservoir at least the volume of the Arctic Ocean.

This reservoir, called the "Beijing anomaly" lies at the top of the lower mantle beneath eastern Asia.

According to the write up by Tony Fitzpatrick at the Washington University in St. Louis web site, it is the first evidence for water existing in the Earth's deep mantle.

3-D seismic model of the Earth`s mantle
(Click for larger image)

Michael E. Wysession, Ph.D., Washington University professor of earth and planetary sciences in Arts & Sciences, working with former graduate student Jesse Lawrence (now at the University of California, San Diego), analyzed 80,000 shear waves from more than 600,000 seismograms and found a large area in Earth's lower mantle beneath eastern Asia where water is damping out, or attenuating, seismic waves from earthquakes.
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In analyzing the data, Wysession first saw large patterns associated with known areas where the ocean floor is sinking down into the earth. Beneath Asia, the fallen Pacific sea floor piles up at the base of the mantle. Right above that he observed an "incredibly highly attenuating region, that is both very damping and slightly slow," he said. "Water slows the speed of waves a little. Lots of damping and a little slowing match the predictions for water very well."

Previous predictions calculated that a cold ocean slab sinking into the earth at 1,200 to 1,4000 kilometers beneath the surface would release water in the rock that would escape the rock and rise up to a region above it, but this was never previously observed.

But those of us seeing an enormous cavern with the waves of an underground ocean lapping its shores, must still settle for something a tad more prosaic.

Just as oil fields are not huge pockets of floating oil, but hydrocarbons soaked into the porous rock, the water reservoir in the mantle is tied up in the rock strata.

"That is exactly what we show here, the exact depth and high attenuation amounts right above it," Wysession said. "I call it the Beijing anomaly. Water inside the rock goes down with the sinking slab and it's quite cold, but it heats up the deeper it goes, and the rock eventually becomes unstable and loses its water. The water then rises up into the overlying region, which becomes saturated with water.

"If you combine the volume of this anomaly with the fact that the rock can hold up to about 0.1 percent of water, that works out to be about an Arctic Ocean's worth of water."
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Seventy percent of the earth is covered by water, which is very important for the earth's geology, serving as a lubricant that allows efficient convection and plate tectonics and the continental collisions that form mountains.

"Water is like a lubricant, constantly oiling the machine of mantle convection which then drives plate tectonics and causes the continents to move about Earth's surface," Wysession said. "Look at our sister planet, Venus. It is very hot and dry inside Venus, and Venus has no plate tectonics. All the water probably boiled off, and without water, there are no plates. The system is locked up, like a rusty Tin Man with no oil."