This is Olympus Mons.
As far as we know, it's the largest mountain--and volcano--in the solar system. At 27,000 m (88,500 ft) high, this BAMF of igneous rock is more than three times the height from sea level of Mount Everest, and more than two and a half times taller than Mauna Kea. On Earth, it would reach well into the stratosphere. The caldera at its summit is about 85 km (53 mi) long by 60 km (37 mi) wide, and up to 3 km (1.8 mi) deep.
And now, you can check it out in 3D.
On Tuesday, the European Space Agency published photos from the Mars Express, an orbiting satellite carrying the High Resolution Stereo Camera (HRSC), which has been taking 3D pictures of the planet's surface for the past three years. Until now, images of Mars have only come from 2D cameras either orbiting or on the surface, and so scientists have had little information on the true contours of the terrain. From these new pictures, the ESA was able to produce a Digital Terrain Model, or topographical playground, to give some perspective (literally) on the proportions of things on the Red Planet.
If you want to learn more about how they obtained the pictures, check out the ESA's website on the Mars Express HRSC. One of the best places to go walk around Mars and get pictures like this one is through the HRSC page presented by the Planetology and Remote Sensing Department, Institute of Geosciences, Freie Universität, Berlin. For tips on navigating around the maps, hit up the "How to use HRSCview" in the left hand navigation.