An article at BoingBoing makes note of the vast quantities of water stored in the earth's mantle.
Deep inside the mantle, where the temperature and pressure are so high you would think it impossible, viscous crystalline rocks potentially trap the equivalent of the Pacific Ocean...See also a previous post on this subject.
We know that ringwoodite can hold water, but it has been determined that below 660 km, ringwoodite transitions into yet another form of olivine called bridgmanite, which can't hold much water. However, seismic mapping experiments have detected areas of melt, melted material held within the crystalline solids that differ in their chemical composition, and which are possibly indicative of water, at depths of 760 km. This is 100 km deeper than water should be able to venture...
Whereas previous estimates have put the amount of water in the mantle at 1-3 times the amount of water on the surface, this study brings that quantity down to a single ocean. Regardless of the reduction, this is still substantial considering that all of the water could have originated from geochemical processes alone.