http://www.newsweek.com/oceans-are-being-sucked-earths-interior-through-worlds-deepest-trenches-1215014

Three times more water is being sucked into Earth’s interior through its subduction zones than was previously thought, scientists have discovered. These huge fissures that scar the planet’s surface appear at the boundaries of tectonic plates. When they collide, they drag water down miles and miles, locking it up inside the rocks in the plates.

Understanding the global water cycle is hugely important—water covers most of the planet and is necessary to the survival of all life on Earth. We know there are vast quantities of water locked up inside Earth. Research published last year indicates there is as much water in Earth’s mantle as there is in all the oceans combined. After being pulled in, the water is eventually spewed back out as water vapor during volcanic eruptions hundreds of miles away.