Scientists Discover a Hidden Underground Water Reservoir Bigger Than All Oceans Scientists have discovered a massive underground water reservoir deep within Earth’s mantle that may contain up to three times more water than all surface oceans combined. The water is not liquid but trapped inside a high-pressure mineral structure deep below Earth’s crust. Key Facts: • Located around 700 km beneath Earth’s surface • Water is stored inside a mineral called ringwoodite • Not liquid water, but chemical