https://phys.org/news/2018-11-seismic-reveals-huge-amount-earth.html

"People knew that subduction zones could bring down water, but they didn't know how much water," said Chen Cai, who recently completed his doctoral studies at Washington University. Cai is the first author of the study published in the Nov. 15 issue of the journal Nature.

"This research shows that subduction zones move far more water into Earth's deep interior—many miles below the surface—than previously thought," said Candace Major, a program director in the National Science Foundation's Division of Ocean Sciences, which funded the study. "The results highlight the important role of subduction zones in Earth's water cycle."