There are areas of Mars that have experienced some striking geological activity since the early days of bombardment. Perhaps most spectacular are the volcanoes of Mars - many of which dward those on Earth. The area called "Tharsis Bulge" in the northern hemisphere hosts an impressive array of volcanic mountains. Four of Tharsis's volcanoes are so tall that their summits are vitaully in space - they protrude above the tops of the occasional global dust storms that occur on Mars.
The most impressive mountain in the Tharsis region is called Olympus Mons - almost known as Mount Olympus. This huge cone is about 300 miles (500 kilometers) across at its base. If it were on Earth it would cover the entire state of Arizona. Its crater or caldera as the depressions on the tops of volcanoes are called, is 80 kilometers wide and its summit is 25 kilometers (15 miles) above the surrounding plain. Just think - Mt. Everest is only 9 kilometers above sea level!