ESA/DLR/FU Berlin (G. Neukum) Valles Marineris, stretching over 4000 km long and 200 km wide, and with a dizzying depth of 10 km, it is ten times longer and five times deeper than Earth's Grand Canyon.
Grand Canyon on Mars that is the Biggest in Entire Solar System
Delicious Reddit

OCTOBER 22, 2012

The Grand Canyon may be an awe inspiring sight for us here on Earth, but a new image of the Martian landscape puts it into perspective - with a canyon over 4000m long and 10km deep. The astonishing new images, released today by the European Space Agency, show the Valles Marineris, believed to be the largest canyon in the Solar System.
Read more: Daily Mail