That water had to get in there somehow, and, using analyses of its depth and its water makeup, Pearson suggests that there’s water deep beneath the Earth’s surface—a lot of it.
The finding “confirms predictions from high-pressure laboratory experiments that a water reservoir comparable in size to all the oceans combined is hidden deep in Earth’s mantle,” according to an analysis of Pearson’s findings by Hans Keppler of the University of Bayreuth in Germany.
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Scientists have long been divided about what, exactly, is in the transition zone. We’ve known that much of the upper mantle is made up of olivine, and, as Keppler said, scientists have long thought that Earth contained reservoirs of water deep beneath the crust. But they weren’t sure whether the water existed as low as the transition zone—the area between the upper and lower mantles. While some say that much of the oceans’ water may have originated there, others have said it is likely completely dry.
The Committee to Nuke Elysium 