The Earth's mantle is about 1,800 miles thick and predominantly solid (silicates). It accounts for 84 percent of the Earth's volume.
Rendering: Western University, Ontario, Canada

In his 1864 science fiction novel Journey to the Centre of the Earth, Jules Verne describes Professor Lidenbrock and his two companions raft on an underground sea, which the professor names after himself.

Such a scenario seems ludicrous, but now researchers are saying there could be a major repository of water in the Earth's mantle greater than all the planet's oceans combined.

The scientists are not talking about an oceanic aquifer, rather, rock that is holding molecules of water. Writing in the journal Nature,¹ published on line March 12, 2014, lead researcher Graham Pearson from the University of Alberta, specifies that at depths between 410 and 660 kilometers (250-375 miles), "higher-pressure polymorphs of olivine"?uwadsleyite and ringwoodite"?uhost enough water to comprise up to around 2.5 per cent of their weight."

"The [ringwoodite] sample really provides extremely strong confirmation that there are local wet spots deep in the Earth in this area," says Pearson. The water, however, is "likely to be locked up in specific rocks in a molecular form called hydroxyl."

The researchers also note, ""?(R)? this hydrous transition zone may have a key role in terrestrial magmatism and plate tectonics."

^{1. Hydrous mantle transition zone indicated by ringwoodite included within diamond. Pearson DG, et al. Nature 507, 221–224 (13 March 2014). www.nature.com/nature/journal/v507/n7491/full/nature13080.html}