Scientists at the Jet Propulsion Laboratory (JPL) in Pasadena, California, announced Thursday that water—fast-running and relatively deep—once coursed over Mars's now bone-dry surface, a finding based on the presence of rounded pebbles and gravel near the rover's landing site in Gale Crater.

What's more, the researchers estimate that the water was present for thousands to millions of years.

The finding represents the first proof that surface water once ran on Mars. Planetary scientists have hypothesized that the cut canyons and riverlike beds photographed by Mars satellites had been created by running water, but only now do they have on-the-ground confirmation—and the promise of learning much more about the nature and duration of the water flows.