As wide as Arizona and ringed by cliffs taller than Mauna Loa, the Martian volcano last erupted just 25 million years ago. Summary and Key Points: Mars built the largest volcano in the solar system, and planetary scientists are still not certain it is finished. Olympus Mons rises about 70,000 feet above the Martian datum, nearly two and a half times the height of Mount E... [10496 chars]