Home to giant volcanoes, Mars would be an even more dramatic place if lava still flowed on the surface today. Olympus Mons, the largest volcano in the solar system, would cover the state of Arizona. Most Martian volcanoes are found in the Tharsis region of the planet. While we've never seen any evidence that these volcanoes are still active, scientists would certainly be interested to find any "hot spots" of activity.