The Martian Olympus Mons is the solar system's largest volcanic mountain. At 27 kilometers, it is three times are high as Mount Everest, and the central crater is 80 kilometers across. Its base is 620 kilometers wide. Its gentle slopes define it as a shied volcano, less steep than the more familiar Vesuvius or Mount Fuji on Earth. Olympus Mons was probably still active at some point during the last billion years, making it a youngster among Martian volacanoes.