This is a color-coded topographic map of the far side of the moon. Since the Moon has no air or water, and hence no "sea level", zero altitude is set at the Moon's mean (average) radius, of just over a thousand miles. Altitudes below that use bluer tones, suggestive of oceanic regions on the Earth, and altitudes above that green and redder tones, suggestive of land regions on the Earth (areas shown in gray have no reliable altitude data). As the result, the lunar maria, once thought to be oceans, are shown as though they were oceans, while the lunar terrae, once thought to be highlands, are shown as though they were continents.
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