What's the Tallest Volcano in the Solar System?

The short answer

The tallest volcano we know of in the solar system is Olympus Mons on Mars. NASA puts it at about 13.6 miles (about 72,000 feet or 22 km) tall, roughly 2.5 times the height of Mount Everest above sea level (about 5.5 miles, or 29,032 feet). Exact numbers vary with how you measure, because Mars has no sea level and the volcano's base is enormous; some NASA materials give about 16 miles (25 km) measured from the surrounding plains. Olympus Mons is a giant shield volcano, like Hawaii's volcanoes but far bigger, and NASA describes it as dormant (not erupting today). Sources: NASA Science and NASA's Moon to Mars educator guide.