Google, in it's continuing attempt to map the known universe, has given us Google Mars. http://www.google.com/mars
Anyone been to, or seen a picture of, America's Grand Canyon? At 500 miles long, up to 20 miles across and over a mile deep, it is quite impressive!
Compare it to the Grand Canyon of Mars (Valles Marineris): 1800 miles long, up to 600 miles wide and up to 5 miles deep. Wow, what a crevace!