"A popular view among geophysicists is that large amounts of water are carried from the oceans to the deep mantle in “subduction zones,” which are boundaries where the Earth’s crustal plates converge, with one plate riding over the other.
But now geophysicists led by the University of California, Riverside’s Harry Green, a distinguished professor of geology and geophysics, present results that contradict this view. They compare seismic and experimental evidence to argue that subducting slabs do not carry water deeper than about 400 kilometers."