Olympus Mons Volcano
This close-up image shows Olympus Mons, the largest volcano and tallest mountain on Mars and is also the largest known volcano in the Solar System. It is 16.7 miles (27 km) high and its base is over 372 miles (600 km) across. This volcano is three times as high of Mount Everest on Earth, and its base could cover the state of Arizona. One of the reasons it is so tall is because the gravity on Mars is so much weaker than on Earth. This allows features to grow taller before they collapse under their own weight.
NASA / U.S. Geological Survey