Scientists just made the most groundbreaking discovery! They found what they're calling the sixth ocean on Earth, but it's not on the surface. This massive reservoir of water is hidden deep underground, beneath the Earth's crust. It's trapped in a layer of rock called ringwoodite, which is located about 400 miles down in the mantle. What's even crazier is that this underground ocean holds as much water as all the surface oceans combined. It completely changes our understanding of Earth's water and where it might be hiding!