Earth is home to stunning snow-capped mountains that tower over their surrounding landscapes, but none quite compares to Mars’ Olympus Mons. First photographed in detail by NASA’s Mariner 9 probe in 1971, Olympus Mons (Latin for “Mount Olympus”) is nearly 16 miles tall. For comparison, its most famous earthly competitor — Mount Everest — is only about5.5 miles above sea level. The width of Olympus Mons is just as impressive as its height: Stretching 374 miles across, it’s as big as the entire state of Arizona. Olympus Mons is what’s known as a shield volcano, a type formed as lava slowly spreads out and cools; these volcanoes usually have a low profile and are named for their resemblance to a warrior’s shield.
So how did Olympus Mons get so big? Scientists think a combination of low surface gravity and high volcanic activity allowed Mars’ great shield volcano to grow — over billions of years — beyond anything seen on Earth. And unlike on Earth, where volcanoes form as tectonic plates drift over hot spots of lava, Mars’ plate movement is much more limited, meaning magma can build and build in one spot over a long time. So while summiting peaks like Everest and K2 remains an impressive terrestrial feat, the solar system’s biggest climbing challenge awaits on the red planet.