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There is much debate concerning whether Mars once had water. A. Mars' runoff and outflow channels are large-scale, distinctive features that suggest that large quantities of liquid water once flowed on Mars. B. Although some researchers claim that Mars may once have had oceans, others dispute this, pointing to an absence of evidence or offering alternative interpretations of evidence. C. Various types of images have been used to demonstrate that most of Martian surface contains evidence of flowing water. D. The runoff and outflow channels of Mars apparently carried a higher volume of water and formed more extensive networks than do Earth's river systems. E. There is very little evidence of liquid water on Mars today, and it is assumed that all the water that once existed on the planet is frozen beneath its surface. F. While numerous gullies have been discovered on Mars since 2000, many astronomers dismiss them as evidence that Mars once had liquid water.