Picture : All the earth's water in a single sphere compared to earth

[muriel_volestrangler]

25. However, there may be a lot of water much deeper

Characteristics of Earth’s deep water cycle such as the persistence of water on the planet’s surface and its storage in the planet’s interior are critical factors that affect plate tectonics. However, the influence is reciprocal with plate tectonics controlling the fluxes of water between near-surface and deep reservoirs.

In the March 2012 issue of Physics Today [1], Marc Hirschman and David Kohlstedt explore the history of the deep water cycle as it is linked to Earth’s history, as well as the role of water in Earth’s mantle from the microscopic considerations to the macroscopic consequences. The authors also note that carbon, like water, cycles through Earth’s interior with vast quantities being stored in the mantle and that it has an equally profound impact on Earth’s dynamics.

https://dco.gl.ciw.edu/water%E2%80%94and-beyond%E2%80%94-earths-mantle

Molten rocks deep in the earths interior may be surprisingly wet, Japanese researchers say. From lab experiments, they have concluded there may be more H2O deep underground than in all oceans, lakes, and rivers combined.

http://news.nationalgeographic.com/news/2002/03/0307_0307_waterworld.html

What is most notable, the scientists say, is those areas of high conductivity coincide with subduction zones – where tectonic plates are being subducted beneath the Earth's crust. Subducting plates are comparatively colder than surrounding mantle materials and thus should be less conductive. The answer, the researchers suggest, may be that conductivity in those areas is enhanced by water drawn downward during the subduction process.
...
"Many earth scientists have thought that tectonic plates are not likely to carry much if any water deep into the Earth's mantle when they are being subducted," said Adam Schultz, a professor in the College of Oceanic and Atmospheric Sciences at Oregon State and a co-author on the Nature study. "Most evidence suggests that subducting rocks initially hold water within their minerals, but that water is released as the rocks heat up."

"There may be other explanations," he added, "but the model clearly shows a close association between subduction zones and high conductivity and the simplest explanation is water."
...
"In fact, we don't really know how much water there is on Earth," said Gary Egbert, also a professor of oceanography at OSU and co-author on the study. "There is some evidence that there is many times more water below the ocean floor than there is in all the oceans of the world combined. Our results may shed some light on this question."

http://www.science20.com/news_articles/water_earths_mantle_may_be_responsible_unexpected_conductivity