Valles Marineris, or Mariner Valley, is a vast canyon system that runs along the Martian equator just east of the Tharsis region. Valles Marineris is 4000 km (2500 mi) long and reaches depths of up to 7 km (4 mi)!
For comparison, the Grand Canyon in Arizona is about 800 km (500 mi) long and 1.6 km (1 mi) deep. In fact, the extent of Valles Marineris is as long as the United States and it spans about 20 percent (1/5) of the entire distance around Mars!
The canyon extends from the Noctis Labyrinthus region in the west to the chaotic terrain in the east. Most researchers agree that Valles Marineris is a large tectonic “crack” in the Martian crust, forming as the planet cooled, affected by the rising crust in the Tharsis region to the west, and subsequently widened by erosional forces. However, near the eastern flanks of the rift there appear to be some channels that may have been formed by water.