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Olympus Mons (Latin, "Mount Olympus") is the tallest known mountain in our solar system, located on the planet Mars at approximately 133°W by 18°N.
The central edifice stands 27 kilometres (88,600 feet) high over its base (over three times the height of Mount Everest); it reaches 25 kilometres above the mean surface level of Mars, since it stands in a two-kilometre-deep depression. It is 540 km (335 miles) in width, flanked by steep cliffs, and has a caldera that is 85 km (53 miles) long, 60 km (37 miles) wide, and up to 3 km (1.8 miles) deep with six overlapping pit craters. Its outer edge is defined by an escarpment up to 6 km (4 miles) tall unique among the shield volcanoes of Mars.
Olympus Mons is an apparently extinct shield volcano, the result of highly fluid lava flowing out of volcanic vents over a long period of time, and is much wider than it is tall; the average slope of Olympus Mons' flanks is very gradual. In 2004 the Mars Express orbiter imaged lava flows on the flanks of Olympus Mons that appear to be only 2 million years old, suggesting that the mountain may yet have some ongoing volcanic activity.
The volcano is surrounded by a region known as the Olympus Mons Aureole (Latin, "Circle of Light") with gigantic ridges and blocks extending 1000 km (600 miles) from the summit that show evidence of development and resurfacing connected with glacial activity. Both the escarpment and the Aureole are poorly understood. In one theory this basal cliff was formed by landslides and the Aureole consists of material piled up at the bottom of these landslides.
 Caldera and pit craters on Olympus Mons The Hawaiian islands are examples of similar shield volcanoes on a smaller scale (see Mauna Loa). The extraordinary size of Olympus Mons is likely due to the fact that Mars does not have tectonic plates. Thus, the crust remained fixed over a hot spot and continued to discharge lava, bringing the volcano to such a height.
Olympus Mons is located in the Tharsis bulge, a huge swelling in the Martian surface that bears numerous other large volcanic features. Among them are a chain of lesser shield volcanoes including Arsia Mons, Pavonis Mons and Ascraeus Mons, which are small only in comparison to Olympus Mons itself. The land immediately surrounding Olympus Mons is a depression in the bulge 2 km deep.
The size of Olympus Mons is so great (roughly the size of the American state of Missouri) that a person standing on the surface of Mars would be unable to view the profile of the volcano even from a distance as the curvature of the planet would obscure such detail. The only way to view the mountain properly is from orbit.
In the days before space probes revealed its identity as a mountain, Olympus Mons was known to astronomers as the albedo feature, Nix Olympica.
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