Do you remember the vast ocean Jules Verne wrote about in his novel Journey to the Center of the Earth? Turns out the sea might be more than science fiction fantasy.

Scientists have found a mineral that suggests the existence of a massive reservoir buried more than 250 miles beneath our feet. It could hold as much water as all the world’s oceans combined, according to the study published Wednesday in Nature.

The hunch rests on the discovery of ringwoodite, a water-loving mineral that comes from the transition zone between the planet’s upper and lower layers. Researchers recently found the material in a diamond that they believe was blasted to the surface by a volcanic eruption.

“No one is ever going to run a geological field trip to the transition zone [310 miles] beneath the Earth's surface, and no one is ever going to drill down to the transition zone," Graham Pearson, a geologist who studied the stone, told the Guardian. "It was a total piece of luck that we found this."

Even though the mysterious underground reservoir is out of reach, humans are still making plenty of new discoveries in our more accessible oceans. According to one study, there are 10 times more deep-sea dwellers living in the “twilight zone” than scientists originally thought. That’s a lot of fish.