The largest canyon known to humanity is on Mars.

Valles Marineris is about 2,500 miles long and would extend across the entire United States if on Earth! In places, this canyon is over 6 miles deep - almost 7 times deeper than the Grand Canyon. This massive structure is said to have been formed by a combination of shifting fault lines and erosion, likely by water.

There are images of the canyon created by merging images taken by the Viking probes with topographic maps. You can take a look at these images here. Some of these are 3-D images, so you will need to wear 3-D glasses (the old-school red and blue kind).