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Olympus Mons is a shield volcano — the same broad, gently-sloped type as the Hawaiian volcanoes here on Earth. But on a scale that has no comparison. It rises 21.9 kilometers above the Martian surface. That's nearly two and a half times the height of Mount Everest above sea level. And it is so wide — 600 kilometers across — that it would cover the entire state of Arizona. And at its base, it is surrounded by a cliff. An escarpment up to 8 kilometers high. Eight kilometers. Taller than any mounta