The 10-km deep canyons of Valles Marineris offer open-windows on the geological history of Mars. The MOC camera revealed the incredible complexity of this history while imaging the walls of these canyons. Layers after layers can be seen. Their exact origin is unknown, but considering the regional history, they are likely made of lava flows, lacustrine sediments, pyroclastic flows, and ash deposits. They tell also that the history of Mars was much more complex than previously thought.