Deep below Earth’s surface—about 400 miles down—scientists have discovered a hidden reservoir of water. It’s not liquid, but locked inside a blue mineral called ringwoodite, found in the mantle under intense pressure.

This “underground ocean” could hold three times more water than all surface oceans combined. In 2014, researchers found ringwoodite inside diamonds pushed up from the deep, and it contained water—hinting at this vast subterranean store.

Why does it matter? This hidden water may help drive plate tectonics and volcanic activity, and some believe it could even be the original source of Earth’s oceans—possibly delivered by ancient, water-rich asteroids.