Subduction zones are places where a slab of oceanic lithosphere plunges into the mantle below.  The slab consists of the sediments on top, crustal rocks in the middle, and the lithospheric mantle on the bottom, all plunging down together as a kind of sandwich.  In each of these layers is an ingredient that plays a key role in shaping the evolution of the Earth over geological time – and that is water.
Geoff Abers has conducted extensive research on water in subduction zones.  In this episode, he explains how he uses seismic observations to map the distribution of water in subducting plates and in the overriding mantle.  He then couples these observations with computer-based models of the physics and chemistry of the subducting plates to predict the fate of the water in the downgoing plate.  The results are surprising — over geological time, the amount of water in all of today’s oceans may have been mixed into the deep mantle by subducting plates.  
Geoff Abers is Professor of Geological Sciences at Cornell University.