Orbital data shows features clearly attributable to flowing liquids: riverbeds, tributary structures, and deltas leading to low-altitude areas, for example. From the sides of some steep craters, images show tracks as if water had burst through the crust, then flowed out, and then froze or evaporated.

In 2005 additional evidence was shown that suggests the existence of a vast frozen sea of water ice below the surface. Doppler mapping technology on Mars or-biters—similar to those used by weather satellites orbiting Earth, but adapted for underground investigation—was used to find a body of ice, ranging in depth from a few feet to several hundred feet, that covered an area larger than the states of New York, New Jersey, Pennsylvania, Ohio, and Indiana combined.