A satellite orbiting Mars has discovered evidence that a giant crater was once a water-filled lake. The discovery was made by the European Space Agency’s Mars Express satellite. The rare find was revealed by the presence of a delta, where flowing water has deposited sediment in a characteristic fan shape. The delta is in the Eberswalde crater, in the southern highlands of Mars. Scientists think it was formed more than 3.7 billion years ago by an asteroid that slammed into the planet. Enough of Ebserswalde was preserved, however, that the telltale forms of the 44 square-mile (115 square km) delta can be seen. Near the top of the crater, the thin, squiggly lines represent feeder channels that would have carried water and sediment. Overall, the delta features paint a picture of a once-full lake filling the crater, providing clear signs that the surface of Mars once flowed with liquid water.