Scientists have discovered that a large reservoir of water may be trapped hundreds of miles beneath the surface.

The body of water is enough to fill the Earth's oceans three times over, according to a study published this week in the journal Science. The water is locked up in a mineral called ringwoodite some 400 miles beneath the crust of the Earth.

 Geophysicist Steve Jacobsen from Northwestern University in the US co-authored the study said that it suggests Earth's water might have come from within, driven to the surface by geological activity, rather than being deposited by icy comets hitting the planet as previously believed.

"Geological processes on the Earth's surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight," Jacobsen said, according to a Northwestern University press release. "I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades."

Jacobsen and his colleagues are the first to provide evidence that there could be water in an area of the Earth's mantle known as the transition zone.

They based their findings on a study of the underground region, which extends across most of the interior of the U.S.

Ringwoodite acts like a sponge thanks to a crystal-like structure that makes it draw hydrogen and trap water, according to the study.

Along with Jacobsen's lab experiments, the study also used data from the USAarray, which is a network of seismometers across the U.S. that measures the vibrations of earthquakes.

The discovery is noteworthy because most melting in the mantle was previously believed to occur at a much shallower distance.

 "If (the stored water) wasn't there, it would be on the surface of the Earth, and mountaintops would be the only land poking out," Jacobsen said.

See Now: OnePlus 6: How Different Will It Be From OnePlus 5?