Water, Water Everywhere

There is a very interesting report (http://www.theguardian.com/science/2014/jun/13/earth-may-have-underground-ocean-three-times-that-on-surface) that a volume of water, some 3 times that in all the world's oceans, may be trapped in the Earth's mantle in a water-trapping mineral called Ringwoodite (after its discoverer). This may be one reason why the oceans have remained relatively constant over millions of years and casts doubt on the idea that our seas were generated by icy comets colliding with the forming planet.