Choosing a land feature that sprawls across a quarter of Mars might seem like cheating, but it's our list, so we make the rules. Sure, we could have chosen Olympus Mons, a shield volcano the size of Arizona and three times the height of Mount Everest, but how would exploring that monster be any easier? Or, continuing eastward past its three smaller sisters in the Tharsis Montes group -- which range from 210-270 miles (350-450 kilometers) across and rise 9 miles (15 kilometers) above their surroundings -- we could shoot down the Valles Marineris. Longer than the United States is wide, the grandest canyon of all girdles one-fifth of the planet, running more than 2,500 miles (4,000 kilometers). Indeed, its 120-mile (200-kilometer) width and 4.3-mile (7-kilometer) depth make our Grand Canyon look like a side gulley [sources: Miller; NASA; NASA].
But wait, we hear you say into your subspace communicator, doesn't this constitute a series of landmarks? Not according to some theories about the region's history, which argue that the Tharsis formation is actually one gigantic volcano (based on a broader definition that includes underlying tectonics and magma flows). Under this model, even the titanic Olympus Mons is but a wart on the face of the solar system's vastest volcano [source: Fazekas].