Olympus Mons ejected material on Mars, illustration. Olympus Mons is the largest volcano in the solar system. North of its mountain is a rough tongue called Lycus Sulci, a valley with some craters called Diacria Patera and a bow in the north, the Archeron Fossae. The ring on the 550 kilometres diameter hill in the lower right corner is the Olympus Mons caldera, which was dtill active some ten million years ago.