Valles Marineris is a vast system of canyons on Mars, located just south of the Martian equator. Valles Marineris (The Valley of the Mariners) is about 4,000 kilometers long – big enough to stretch all the way across the United States. The central individual troughs, generally 50 to 100 kilometers wide, merge into a depression as much as 600 kilometers wide. In places the canyon floor reaches a depth of 10 kilometers, 6 to 7 times deeper than the Grand Canyon.

The geologic history of Valles Marineris is complex: first the surface collapsed into a few deep depressions that later became filled with layered material, perhaps as lake deposits. Then graben-forming faults cut across some of the older troughs thus widening existing troughs, breaching barriers between troughs, and forming additional ones. At that time the interior deposits were locally bent and tilted, and perhaps water, if still present, spilled out and flowed toward the outflow channels. Huge landslides fell into the voids created by the new grabens. Wind-drifted material, mostly dark in color, apparently still moves along the canyon floor and locally forms conspicuous dunes.