The discovery might explain where all our oceans water cam from.

"Geological processes on the Earth’s surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight," geophysicist Dr. Steven Jacobsen, an associate professor at Northwestern University, said in a written statement. "I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades." 

The hidden reservoir, is  locked in a blue crystalline mineral called ringwoodite.