Olympus Mons is the largest volcano in the solar system. The massive Martian mountain towers high above the surrounding plains of the red planet, and may be biding its time until the next eruption.
Found in the Tharsis Montes region near the Martian equator, Olympus Mons is one of a dozen large volcanoes, many of which are ten to a hundred times taller than their terrestrial counterparts. The tallest of them all towers 16 miles (25 kilometers) above the surrounding plains and stretches across 374 miles (624 km) — roughly the size of the state of Arizona.
In comparison, Hawaii's Mauna Loa, the tallest volcano on Earth, rises 6.3 miles (10 km) above the sea floor (but its peak is only 2.6 miles above sea level). The volume contained by Olympus Mons is about a hundred times that of Mauna Loa, and the Hawaiian island chain that houses the Earthly volcano could fit inside its Martian counterpart.