Olympus Mons is the largest volcano discovered so far in the entire solar system, and is part of the Tharsis volcano area on Mars. The behemoth rises 25km (15.5 miles) high and stretches over 550 km (340 miles) from east to west. The summit caldera is a composite of at least seven circular collapsed depressions. The caldera itself if 66 by 83 km (41 by 52 miles) across.   (11/53)