A vast ocean once covered a third of Mars, scientists believe.
Such a stunning prospect greatly increases the chances of life having existed on the Red Planet, the fourth from the Sun in our solar system.
Researchers have come to the conclusion after using new software to analyse images of the surface. As a result, they have managed to find dozens of valleys to build up the most detailed map to date.
The valleys, first spotted in 1971, were caused by a network of rivers more than twice as extensive as previously mapped. The water channels were in a belt between the equator and mid-southern latitudes.
The experts from Northern Illinois University and Nasa believe they mark the paths of rivers that once flowed from the planet's southern highlands into a huge ocean in the north.