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Olympus Mons

The northwest side of Olympus Mons' 20,000 foot scarps cast long shadows into the water mist and dust over the plains of the Tharsis Bulge. High above, the brightest object in the black sky is Mars' largest satellite Phobos.

Nearly as large as the state of Arizona and three times the height of Mount Everest, Olympus Mons is the largest volcano--and mountain--in the Solar System. It is thought to be very old, though its last eruption may have been as (geologically) recent as 40 million years ago. It may yet still be active.
In Wikipedia: http://en.wikipedia.org/wiki/Olympus_Mons