This 3-dimensional image was created from several images of Olympus Mons. Each image was taken from a different spacecraft position and combined with a computer model of the surface topography to generate a perspective view. The final mosaic shows Olympus as it would be seen from the northeast. It's possible that volcanoes of such magnitude were able to form on Mars because the hot volcanic regions in the mantle remained fixed relative to the surface for hundreds of millions of years.
Source: NASA JPL