This is the Valles Marineris, believed to be the biggest canyon in the entire solar system. At nearly 2,500 miles long, it would stretch clear across the continental United States.
Much like Mars's highest peak, Olympus Mons, the sheer scale of Valles Marineris is almost incomprehensible. Depending on how you measure it, its total length falls somewhere between 3000 and 4000 kilometers, its width is as much as 600 kilometers, and it reaches 8 kilometers deep at points.
A NASA astronomer adds:
By comparison, the Earth's Grand Canyon in Arizona, USA is 800 kilometers long, 30 kilometers across, and 1.8 kilometers deep.The origin of the Valles Marineris remains unknown, although a leading hypothesis holds that it started as a crack billions of years ago as the planet cooled. Several geologic processes have been identified in the canyon. The above mosaic was created from over 100 images of Mars taken by Viking Orbiters in the 1970s.
For more on Valles Marineris and other extreme points in the solar system - and how to climb them - check out our earlier feature on solar system mountaineering.