In September Curiosity found an outcrop of conglomerate rock, pebbles and gravel bound together by wet sediment. After years of inferring the existence of ancient water from chemical analyses and satellite photos, this is by far the most direct proof that water once flowed on Mars. Scientists estimate that 4 billion years ago a river up to a few feet deep lazily flowed down from Gale Crater's rim. Curiosity's next task is to determine whether other conditions on ancient Mars, like the chemistry of the soil and air, were also favorable to the development of microbial life.