A reservoir of water three times the volume of all the oceans has been discovered deep beneath the Earth's surface. The finding could help explain where Earth's seas came from.

The water is hidden inside a blue rock called ringwoodite that lies 700 kilometers underground in the mantle, the layer of hot rock between Earth's surface and its core.

The huge size of the reservoir throws new light on the origin of Earth's water. Some geologists think water arrived in comets as they struck the planet, but the new discovery supports an alternative idea that the oceans gradually oozed out of the interior of the early Earth. [New Scientist]