Deep-sea fault zones bringing water from the Earth's seas to the upper mantle could be creating an ocean beneath our feet, British researchers say. Water is carried into the mantle by deep-sea fault zones that penetrate the oceanic plates as they bend into subduction zones, where an oceanic tectonic plate is forced beneath another plate, they said. As the tectonic plates heat up, this water causes the mantle material to melt, causing volcanoes above the subduction zone such as those that form the Pacific "ring of fire," researchers at the University of Liverpool said. University seismologists at Liverpool have estimated that over the age of the Earth, the Japan subduction zone alone -- site of the 2011 earthquake that caused a devastating tsunami -- could transport the equivalent of as much as 3 1/2 times the water of all the Earth's oceans to its mantle. "This water causes melting in the mantle, which leads to arc releasing some of the water back into the atmosphere," doctoral student Tom Garth said. "Part of the subducted water however is carried deeper into the mantle and may be stored there." Understanding how much water is delivered to the mantle adds to the knowledge of how the mantle convects and melts and how that is involved in earthquakes and volcanic eruption, the researches said.