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.

Researchers from Northwestern University and University of New Mexico have found deep pockets of magma located beneath North America, a likely signature of the presence of water at these depths.

This water is not in a form familiar to us - it is not liquid, ice or vapour.

The discovery suggests water from the earth's surface can be driven to such great depths by plate tectonics, eventually causing partial melting of the rocks found deep in the mantle.