.  A study published in the journal Science suggests a hidden "ocean" in the Earth's mantle, some 650 kilometers (400 miles) beneath North America. This vast reservoir of water may be locked in a blue crystalline mineral called ringwoodite. The evidence comes from seismic waves that ripple through Earth following earthquakes. This evidence suggests to these scientists that the ringwoodite in Earth's mantle is saturated with liquid. They believe there might be three times as much water underground as exists in all the world's surface oceans.
    The scientists—geophysicist Dr. Steven Jacobsen at Northwestern University and seismologist Dr. Brandon Schmandt of the University of New Mexico—so far only have evidence that the water saturated rock sits beneath North America. They want to know if it wraps around Earth.
    If it does, it should help scientists understand the processes that govern life on Earth since water is, literally, life. On our planet, everywhere that we have found traces of water, we have found life. This makes the latest discovery by geologists extremely exciting.1.  What is the so called hidden "ocean" according to the passage? ______