Scientists have long suspected that there is water trapped deep inside the Earth's interior. However, they had never been able to verify it. Hence geologists had no choice but to adopt the theory that the water in our oceans was deposited by icy comets hitting the forming planet. Now thanks to evidence uncovered by Associate Professor Steven Jacobsen of Northwestern University and seismologist Brandon Schmandt from the University of New Mexico, there is finally proof that our oceans may have emerged from a massive reserve that lies about 400 miles beneath the Earth's crust.

The two scientists use different research techniques - Schmandt's team studied the seismic waves of 500 earthquakes collected by the USArray's network of 200 seismometers that have been placed across the United States. The team noticed that the speed of waves, which differed at various depths, slowed down significantly as they passed through ringwoodite, a bright blue rock which lies in the transition zone - the area between the upper and lower mantle. This led them to suspect that it was due to the moisture in the rocks.

Jacobsen's team reached the same conclusion. However in their case it was by growing the mineral in the laboratory and subjecting it to the same pressure and temperature as it would face 400 miles beneath the earth's surface. Sure enough, the water squeezed out of the ringwoodite as if the rock was sweating.

Schmandt describes it as dehydration melting and says that "When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit,". The water from the melted portion gets trapped in the transition zone, thanks to the crystal structure of ringwoodite which acts like a sponge and stores it, creating a reservoir.

However, thanks to the pressure coupled with the high temperature, the water in these rocks is not in the three forms - liquid, ice, or vapor - that we know of. Instead, it splits into hydoxyl radical (OH) that is bound inside the ringwoodite's mineral structure.

which can be bound into a mineral’s crystal structure. - See more at: http://www.northwestern.edu/newscenter/stories/2014/06/new-evidence-for-oceans-of-water-deep-in-the-earth.html#sthash.UzmKHzul.dpuf

These findings have led the scientists, who published their joint study in the June 13th edition of Science, to conclude that our oceans are probably the result of geological activity pushing out some of the large reserves that lie underneath. This may also explain why they have stayed roughly the same size for millions of years: the hidden water acts as a buffer from the inside.

Schmandt and Jacobsen's theory is supported by another study published in the March 2014 edition of Nature by a different group of researchers. They were able to examine a natural ringwoodite sample, the only one of its kind, that was coughed up by a volcano in 2009. They discovered that about 1.5% of the rock's weight is water, almost identical to what Jacobsen had deduced from his laboratory experiments.

While Schmant's research only confirmed the presence of the soggy ringwoodite beneath the continental United States, he suspects it may be prevalent throughout the planet. Given how vast Earth's mantle is, even the presence of just 1% would mean a reservoir that is three times larger than all the oceans combined or about 986 million trillion gallons! As Jacobsen puts it "if it wasn't there, it would be on the surface of the Earth, and mountain tops would be the only land poking out".

Resources: newscientist.com,sciencemag.org,npr.org