Mars - Tharsis region. Tharsis is a volcanic plateau in Mars' western hemisphere. It is home to several volcanoes, including Olympus Mons (bottom right), the largest volcanic feature in the Solar System. On the left limb of the planet can be seen the three smaller but still substantial extinct volcanoes Arsia Mons (bottom), Pavonis Mons (middle) and Ascraeus Mons (top). These three volcanoes range in size from 350 to 450 kilometres (km) across and they each have a height of approximately 15 km.