The large body of water was fed through rivers carrying rainwater, scientists believe.
These created a network of valleys on the surface of the planet more than twice as extensive as previously thought, new research reveals.
The findings come just a week after Nasa, the American space agency, announced that they had found water on the surface of the Red Planet, raising hopes of finding life on Mars.
New maps showing that the valleys cover a larger area than previously appreciated has led scientists to believe there was once a single ocean covering much of planet’s northern half.
The extent of the Martian valleys, and what they mean for the chances of life on the planet, have been hotly debated since they were first discovered by the Mariner 9 Spacecraft in 1971.