Nirgal Vallis is morphologically similar to terrestrial sapping valleys that form by headward erosion and undercutting triggered by groundwater seepage. The absence of a dendritic valley network feeding the main valley argues against precipitation and overland flow. With the help of scaled sandbox experiments scientists showed that a sapping origin of Nirgal Vallis is a possibility, if it was fed by a groundwater source. Sapping valleys on Earth, attributed to undermining and collapse of the walls by seepage, mostly occur in unconsolidated sediments with a permeable aquifer.