Mars once had an ocean's worth of water. Today, most of Mars is as dry. Where did the rest of the water go?

Some of it disappeared into space. But most of the water, a study concludes, was sucked into rocks. And there it remains, trapped within minerals and salts. As much as 99% of the water that once flowed on Mars could still be there, the researchers estimated in a paper in the journal Science.

Data from the past two decades showed a wide distribution of what geologists call hydrated minerals.

"It became very, very clear that it was common and not rare to find evidence of water alteration," said Bethany Ehlmann, a professor of planetary science at the California Institute of Technology. "Water is effectively trapped into the crust."

To get a sense of the amount of water, planetary scientists talk about a "global equivalent layer" — that is, if Mars were smoothed out into a ball, how deep would the water have been? The most likely depth was about 2,000 feet, they said, or roughly one-fourth as much water as is in the Atlantic Ocean.