A large upland region on Mars over 2000 km across, centred at +2° lat., 113° W long.; also known as the Tharsis ridge. It includes the three giant volcanoes Ascraeus Mons (diameter 456 km), Pavonis Mons (367 km), and Arsia Mons (470 km), each of which rises to about 20 km high. Tharsis is over 9 km high at the bases of these volcanoes and reaches over 11 km high at Noctis Labyrinthus, a complex network of fractures at the western end of the Valles Marineris.