NASA's Curiosity rover has found evidence that a stream of water may have once ran across the Martian surface. While there has been prior evidence of water on Mars, this evidence is the first of it's kind. Since the two months Curiosity has been on the Red Planet, it has seen some amazing things, including three particular outcrops named the "Goulburn," "Link," and "Hottah."
What is peculiar about these outcrops is the types of rocks being found. Instead of the usual jagged and angled rocks scattered across the Martian surface, there are many smoothed and rounded pebbles and sand grains. This is a tell-tale sign of running water, as stones carried by running water typically become rounded from being battered against each other.