NASA - Mars Odyssey Imagery: "A 'Grand Canyon of Mars' slices across the Red Planet near its equator. This canyon -- Valles Marineris, or the Mariner Valley -- is 10 times longer and deeper than Arizona's Grand Canyon, and 20 times wider. As the picture shows, you could drop the whole Los Angeles basin into a small part of Valles Marineris and leave plenty of room to spare. In length, the canyon extends far enough that it could reach across the United States from East Coast to West Coast, while its rim stands more than 25,000 feet high, nearly as tall as Earth's Mount Everest."
Several years worth of imagery and altimeter measurements have been combined into a 3D representation of Valles Marineris. This has produced the latest flythrough of the canyon.