A groundbreaking new study proves that Martian pebbles traveled at least 50 kilometers from their source, meaning that huge, roaring rivers of liquid water once existed on the Red Planet.

Far from being a few babbling brooks, the study finds that Mars had an extensive river system, a far cry from what it is today: a largely barren rock with a little bit of liquid water, according to a The Indian Panorama report.

The rivers were in existence approximately three billion years ago, based on images from the Mars Curiosity rover sent back to Earth in 2012. The images captured small, smooth pebbles in what appears to be an ancient riverbed.

Researchers from the University of Pennsylvania and Budapest University of Technology and Economics were able to estimate the distance river pebbles traveled based simply on their shapes. These pebbles must have traveled about 50 kilometers from a source, and the only way that could have happened was by flowing waters.

The researchers noted that an object’s shape can say a lot about what it is, and using that information they were able to deduce the origin of these pebbles. As rocks flow in a river, they bounce against other rocks and start to smooth into a rounder shape. Scientists were able to go deeper, figuring out geometrically just how much each individual rock was influenced by other rocks. They tested this method in the lab by rolling limestone fragments in a drum and then recording their shape changes and the mass that was lost. This experiment confirmed their findings.

They went further, going to a mountain river in Puerto Rico and found a trend between the evolution of a rock’s shape and the amount of mass it had lost that seemed to agree with this geometric model.