The largest mountain in the Solar System according to current astronomical observations, Olympus Mons on the planet Mars is a shield volcano almost three times the height of Mount Everest (25km) and enormously wide, 550km along its east-west diameter. It is the largest of the Martian volcanic chain called the Tharsis Montes, the others being Arsia Mons, Pavonis Mons and Ascreus Mons. Olympus Mons is surrounded by a 4km high escarpment, and at its summit is a volcanic caldera which is 83km across at its widest point, formed by seven separate collapses, which were probably not the result of eruptions but rather weaknesses in the roof of deep chambers of magma.
The huge size of Olympus Mons, and the other Martian volcanoes, has been attributed to the low gravity which allows volcanoes to rise slowly in height over a great period of time (Olympus Mons is now so tall that it protrudes from the atmosphere). Because it is a shield volcano, its eruptions are slow and basaltic, not spewing chunks of rock thousands of feet in the air, but rather oozing lava flows slowly over time. Olympus Mons, though very tall, is so wide that its angle rises by only a few degrees (between 2 and 5 in most places), which means it wouldn't be much of a challenge to the mountaineers among any future planetary colonists. Beyond the escarpment is a lava moat, and beyond that is the Olympus Mons aureole, a large flat plain covered by characteristic grooves which probably result from the mountain's volcanic activity.
Highly detailed pictures of Olympus Mons and its surrounding area were recently taken by the Mars Global Surveyor satellite (http://nssdc.gsfc.nasa.gov/planetary/marsurv.html).