Water was common across a vast region of ancient Mars, creating habitable conditions for long stretches of time billions of years ago, scientists said Thursday. New data reveal water in the Meridiani Planum region of Mars extended across hundreds of thousands of square miles, at least as groundwater and possibly as shallow lakes or seas. The work significantly expands the amount of surface area on Mars known to have once been water-laden, and it extends the period of time that the water was present.