Flexi Says: Dao Vallis is a large landform on the surface of Mars, shaped like a river channel approimately 1200km (750 miles) long. Dao Vallis extends southward of Hadriacus Mons (a volcano). Scientists have hypothesized that the channel was formed by rushing water that was formed when volcanic activity melted subsurface ice. The water moved to the surface and flowed downward from the volcano. An alternate hypothesis has been that Dao Vallis was formed by lava flows rather than water. Scientists will look for more detailed evidence of alluvial deposits to confirm which hypothesis is correct.