The canyon was discovered during, and named for, the Mariner 9 mission in 1971.
Valles Marineris is a large tectonic “crack” in the Martian crust, forming as the planet cooled. The canyon is more than 2,500 miles long, 120 mi wide, 23,000 ft deep. It is as long as the United States and it spans about 20 percent (1/5) of the entire distance around Mars. For comparison, the Grand Canyon in Arizona is about 500 mi long and 1 mile deep.
The vast canyon can be seen from Earth through a telescope as a dark scarring on the planet’s surface.