Researchers from Northwestern University and the University of New Mexico found evidence of pockets of melting rock about 400 miles beneath North America, a likely sign of water at that depth. It is not an underground ocean: the ingredients for water are bound inside minerals in the mantle's transition zone, but the amount may make it the planet's largest water reservoir.
Steve Jacobsen synthesised the blue mineral ringwoodite, which can hold more than 1% of its weight as water, and found it partly melts under the conditions 400 miles down. Brandon Schmandt matched this with seismic data from more than 2,000 seismometers across the United States. The study was published in Science in June 2014.
via New Evidence for Oceans of Water Deep in the Earth | Northwestern Now