The largest volcano in the Solar System, lying on Mars at +19° lat., 134° W long. From Earth, it is visible as a white spot previously known as Nix Olympica. The summit of Olympus Mons is 21 km above the mean surface level of Mars, and contains a caldera 80 km across. From the caldera, several lava flows have issued down to a scarp about 4 km high which surrounds the volcano. The diameter of Olympus Mons at the scarp is about 610 km. Around the scarp are many ancient landslips, themselves the largest known in the Solar System, some of which have produced an apron of debris extending out another 600 km or more. The whole complex of volcano and scarps is in places 1600 km across. Olympus Mons is thought to be comparatively young, and was probably active within the last billion years.

http://www.esa.int/spaceinimages/Images/2008/02/Olympus_Mons_ortho-image