Is All the Water in the Oceans Just a Minor Part of Earth's Water? 

Another super correspondent (not the Mars and teeth one) gave me the link to THIS ARTICLE. It describes the minerals likely to be found in the transition between the upper and lower mantle - wadsleyite and ringwoodite. These do not ordinarily exist on the earth's surface. They can, theoretically, hold water. The speed of seismic waves through them is affected by their water content. The author thought that this proved that the transition zone was water-rich, but refrained from publishing as to suggest such a thing was somewhat radical.

Then a crystal of ringwoodite was found inside a Brazilian diamond formed in the transition zone and it contained 1% water! The amounts of ringwoodite are huge, therefore the amount of water must be huge - twice the volume of the oceans! (No one has quantified this in terms of Olympic sized swimming pools - yet!)

Implications of this are that there must be molten material in the transition zone - some seismic work confirms this.

Further work suggests more water above and below the transition zone.

The article then discusses the presence of water and suggest it is essential for plate tectonics and life on earth.

Geophysicists are finding more and more water deep under Earth’s surface, especially in the transition zone, which is full of waterbearing minerals. (Credit: Jay Smith)