The Grand Canyon of Mars

The ESA Mars Express has some amazing pictures of Hebes Chasma, "an enclosed trough, almost 8000 m deep, in Valles Marineris, the Grand Canyon of Mars, where water is believed to have flowed"

Wikipedia says: "Its maximum extents are approximately 320km east-west, 130km north-south and 5-6km in depth. At the center of the depression there is a large mesa rising some 5km off the valley floor, thus nearly as high as the surrounding terrain. This central plateau makes Hebes Chasma a unique valley in Martian geography."

Many more pics at the mission site.