The 2001 Odyssey spacecraft captured a first-of-its-kind look at Arsia Mons, one of the biggest volcanoes on Mars. The picture shows the volcano poking through a canopy of clouds just before dawn. In the photo, Arsia Mons stands 20 km high, roughly twice as tall as Earth's largest volcano, Mauna Loa, which rises 9 km above the seafloor.