Picture a canyon that stretches from San Francisco to Washington DC, and you'll have an idea of the scope of Valles Marineris on Mars.

This enormous gorge was first spotted by the NASA spacecraft Mariner 9 in 1972, and the canyon was named in the spacecraft's honor. It stretches 2,485 miles across the planet's surface, and reaches depths of 6.2 miles (for comparison, our Grand Canyon plunges 1.1 miles down at its deepest point). Valles Marineris is thought to be a rift valley, formed by uplift when hot material from the Mars's mantle bubbled up and stretched the planet's crust.