In what could quench the thirst of billions of people in the future, researchers have discovered our planet's largest water reservoir 640 km beneath our feet - bound up in rock deep in the earth's mantle. This water is not in a form familiar to us - it is not liquid, ice or vapour. This fourth form is water trapped inside the molecular structure of the minerals in the mantle rock. Scientists have been looking for this missing deep water for decades," explained geophysicist Steve Jacobsen from Northwestern University.