The origin of Vastitas Borealis is controversial. Spacecraft images have made it clear that enormous flood channels, called outflow channels, once transported large volumes of water into this region from the higher elevations to the south. Some scientists have suggested that the smooth, almost level surface of the plain is the result of the deposition of sediments from an ocean formed by the floodwater and covering most of the northern plains; they point to linear features (interpreted as shorelines) and terraces around low hills as supporting evidence. Others question that the level surface and linear features are the result of marine processes; they suggest that the floodwater occupied only a small fraction of the northern plains.