Mount Everest's got nothing on Olympus Mons, the tallest mountain in our solar system. At 15.5 miles high, it's also the largest volcano on Mars, covering the size of Arizona. Smithsonian listed the the top ten tallest mountains in the solar system. Earth barely made the list with Mauna Loa. And you thought Everest was our tallest? It's the highest peak, but mountain height is actually measured from base to peak and Everest's base is way above sea level. "The Tallest Mountains in the Solar System"