The highest peak on Mars, and the largest volcano in the solar system. It rises to a height of 27 kilometres 17 miles above the datum level selected on the basis of atmospheric pressure.This gigantic shield volcano, 700 kilometres 435 miles across, is similar in nature to volcanoes on Earth but its volume is at least fifty times greater than its nearest terrestrial equivalent. The caldera at the summit is 90 kilometres 60 miles across and a cliff at least 4 kilometres high rings the mountain. Older volcanic rocks, fractured and eroded by the wind, surround the main peak, forming an area called the aureole. Olympus Mons is located to the north-west of the Tharsis region and was formerly known as Nix Olympica Olympic Snow because the clouds over the area appeared as a light-coloured spot to Earth-based observers.