The central ediface 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, with a crater that is 85 km (53 miles) in diameter.
Olympus Mons is an apparently extinct shield volcano, the result of highly fluid magma 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. The Hawaiian islands are an example of similar shield volcanoes on a smaller scale; see Mauna Loa. The size of this volcano is likley due to the fact that Mars does not have tectonic activity. Thus, the crust stayed over top of that hot spot and continued to discharge lava, brining it 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 2km deep.
Olympus Mons is located at approximately 133°W by 18°N.