Valles Marineris, the famous martian canyon, is among the biggest geographical features of the solar system : 4000 km long, 200 km wide and up to 7 km deep ! (Wikipedia)
To give an idea of the terrible scale of this place, I've made this new photo-montage showing a rover exploration of Noctis Labyrinthus, a region in the western edge of the canyon.
As an information, the rover you can see here is a 6 m tall vehicle. A man would be half of its wheels height.
(Made with The Gimp on Linux-Ubuntu)