Probably the 'Grandest' canyon in our Solar System
. Valles Marineris (Mariner Valley) stretches 4000 km/2500 miles across the surface of Mars
(taking up almost 20 percent of its surface) and cuts up to 7 km/4 miles deep. As a comparison, 'The Grand Canyon
' in Arizona (U.S) is about 800km/500 miles long and only 1.6 km/1 mile deep.
Filmed by the Mars Global Surveyor and other probes, it's believed to be a large tectonic crack that formed as the planet cooled millions of years ago and probably had many rivers flowing in it when(if) Mars had an atmosphere.
Many images can be found on the internet, including this closeup of a small section ...(yes, thats one URL)