Scientists have found an enormous store of water nearly 700 km below the Earth's surface, inside the mantle, a finding that challenges existing ideas about where Earth's water came from. The water is not liquid; it is chemically locked within ringwoodite, a mineral that forms under high pressure. This part of the mantle is known as the transition zone, lying between 410 km and 660 km deep. Researchers detected it through seismic studies, since waves slow down in water-rich rock, and by examining mineral samples linked to volcanic activity. The reservoir may hold more water than all the oceans combined.