Scientists estimate that Mars has been an arid and dusty planet for at least the last two billion years. But this week, NASA’s Curiosity rover uncovered more evidence that a gigantic lake—possibly the size of Utah’s Great Salt Lake—could have once existed in Gale Crater. Ashwin Vasavada, deputy project scientist for the Curiosity Mission, discusses the clues that are leading scientists to believe that this lake, or series of lakes, may have been on the Martian surface for millions of years.