Scientists find a large reservoir of water inside the Earth with a volume of the Arctic Ocean. They analyzed 600,000 seismic records and found water buried in slabs of rock underneath the city of Beijing.

Scientists scanning the deep interior of Earth have found evidence of a vast water reservoir beneath eastern Asia that is at least the volume of the Arctic Ocean. The discovery marks the first time such a large body of water has found in the planet’s deep mantle.

Michael Wysession, a Seismologist at Washington University at St. Louis and his former graduate student Jesse Lawrence, now at the University of California, San Diego found this discovery and will be reported in a monograph to be published at the American Geophysical Union.

They both analyzed 600,000 seismograms records of waves generated by earthquakes traveling through the Earth and collected from instruments scattered around the planet.

They found at a particular region beneath Asia, near Beijing, China, the Seismic waves seem to dampen or attenuate and slow down slightly. They tried to figure out what causes this damping effect and found that water can slow down the speed of waves.

Wysession explained:
Lots of damping and a little slowing match the predictions for water very well.
Previous predictions calculated that if a cold slab of the ocean floor were to sink thousands of miles into the Earth’s mantle, the hot temperatures would cause water stored inside the rock to evaporate out.

Earth has a central hot core of magma, so any water present will be evaporated quickly. When a slab of rock sinks it carries the water along with it. Initially the water is cold, but it heats up and evaporates the deeper it travels.

This evaporated water then rises up into an overlying region which becomes saturated with water. It is shown below here in the red portions of the image. Most of the red portions are seen in China, Beijing area:

“It would still look like solid rock to you,” Wysession told LiveScience. “You would have to put it in the lab to find the water in it.”
Although they appear solid, the composition of some ocean floor rocks is up to 15 per cent water. The water molecules are stuck in between the mineral structure of the rocks. When these rocks are heated, the water then dehydrates. Wysession says it is like heating the clay and firing it to get all the water out.

The researchers estimate that up to 0.1 per cent of the rock sinking down into the Earth’s mantle in that part of the world is water, which works out to about the equivalent of the volume of water in the Arctic Ocean. Wysession said it is not an exact calculation, but gives an overall estimate of how much water is locked up inside.

Wysession has dubbed this new underground feature as the “Beijing anomaly”. The reason being they found seismic attenuation or damping the highest beneath the capital city of China.
“They thought it was very, very interesting,” Wysession said. “China is under greater seismic risk than just about any country in the world, so they are very interested in seismology.”
Water covers 70 per cent of Earth’s surface and one of its many functions is to act like a lubricant for the movement of continental plates.
“Look at our sister planet, Venus,” Wysession said. “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.”