This image is a large mosaic of the Valles Marineris [VAL-less mar-uh-NAIR-iss] hemisphere of Mars. It is a view similar to that which one would see from a spacecraft. The lower centre of the scene shows the entire Valles Marineris canyon system, more than 3,000 kilometres (1,860 miles) long and up to 8 kilometres (5 miles) deep, extending from Noctis Labyrinthus, the arcuate system of graben to the west, to the chaotic terrain to the east. Many huge ancient river channels begin from the chaotic terrain and north-central canyons and run north. Many of the channels flowed into a basin called Acidalia Planitia, which is the dark area in the extreme north of this picture. The three Tharsis volcanoes (dark red spots), each about 25 kilometres (16 miles) high, are visible to the west along with Olympus Mons the largest volcano on the planet. Very ancient terrain covered by many impact craters lies to the south of Valles Marineris. The polar cap can be seen to the north.