Four hundred miles beneath the United States, there appears to be enough water to fill all our oceans—almost three times. Researchers have for the first time discovered direct evidence of the water, which exists as a fourth form: not liquid, not ice, not vapor, but hydrogen and oxygen molecules trapped in a mineral called ringwoodite found in the transition zone, which sits between the upper and lower mantles, USA Today reports. Their findings follow up on the discovery, announced in March, of a water-rich sample of ringwoodite from the transition zone that was brought to the surface via a Brazilian volcano. That find "showed us at least one place there is water, but it didn't tell us how expansive the water would be," explains Steve Jacobsen, one of the lead researchers. By analyzing how seismic waves moved at various depths, they found that "water is everywhere beneath North America," New Scientist reports.
To measure all of the United States took eight years, and the team wants to next determine whether the water-rich minerals extend around the whole planet, though doing so is reliant on the availability of seismometers in other regions. "Scientists have been looking for this missing deep water for decades," Jacobsen says, according to the Guardian; its existence was first theorized in 1987. The discovery could change our understanding of how the oceans formed: While some scientists have theorized the oceans' water came from a comet or asteroid, these findings suggest the oceans actually hail from water deep inside the planet that oozed out of rocks and was driven to the surface by geological activity in Earth's early days. The underground "reservoir" could hold three-quarters of the Earth's water. (Click for more on the ringwoodite found in a "hell diamond" in Brazil.)