Images of steep-sided gullies, sinuous channels, and deltas of debris have been captured by Mars Global Surveyor, the satellite currently in orbit around the Red Planet.
Water could still be flowing on Mars, Nasa scientists believe.
The announcement follows two days of speculation that evidence exists that recent running water has cut channels into the flanks of craters, something previously considered impossible.
Images of steep-sided gullies, sinuous channels, and deltas of debris have been captured by Mars Global Surveyor, the satellite currently in orbit around the Red Planet.
These suggest that liquid water may be lurking just below the Martian surface, say researchers Dr Michael Malin and Dr Kenneth Edgett in a paper published in the journal Science.
It is a discovery that will change the direction of Mars exploration and boost demands for a lander to be sent to investigate the water and look for signs of life.
"If it is true that there is water on Mars near the surface, it has profound implications for the prospect of life on Mars," said Ed Wieler, with the office of space science at Nasa.
Short-lived torrents
The Science paper does not say that water itself has been detected - only structures that, if found on Earth, would have been formed by water seeping up from underground.
The formations are unblemished by impact craters, freeze cracks, or windblown deposits.
"These gullies could be on the order of a million years old, or they could have formed yesterday," says Dr Malin, who with Dr Edgett works out of Malin Space Science Systems, Inc.
The images are stunning and convincing: banked, winding, and often branching channel paths, with final fans of debris.
They suggest that the water could exist in a porous layer of rock buried a few hundred meters below the Martian surface, kept liquid by the pressure exerted by overlying rock.
Cooler areas
The researchers suggest that the formation of the Martian gullies may be linked to their location. More than 90% of them occur in the planet's southern hemisphere, almost all of them are found on the pole side of 30 degrees latitude.
Water on Mars could be widespread. About 200 of the 60,000 images produced by the Mars Global Surveyor are said to show evidence of grooves carved by rivers flowing down the sides of craters in fan-shaped patterns.
Some scientists will find this discovery unsettling because it does not fit into what is understood about Mars. They know that water must be there but it had been thought that it was in the form of subsurface ice and not running water on its surface.
Liquid water is the key ingredient for life so finding it on the surface represents a significant boost for those who think there may be micro-organisms still living on the Red Planet.
This discovery will set the direction for the exploration of Mars in the future.
In the next few months, scientists will closely examine new pictures from Mars - any changes in the landforms would confirm water is flowing.
In the next few years, placing a lander in the watery region will be what every Mars scientist wants to do.