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.