Utopia Planitia is one of three major topographic basins in the northern hemisphere of Mars and shows a diameter 3,300 km. It is interpreted to represent an ancient impact basin, that was variably filled by a mix of sediments, volatiles and lavas, transported by water, wind and other processes. Ice in the subsurface has been confirmed by spectral measurements of the hydrogen abundance and directly by the Phoenix lander. Ground ice at greater depth has been detected by observations of fresh impact craters and sublimation pits, as well as by radar sounding.