Olympus Mons is the tallest volcano and highest planetary mountain in the Solar System.
It stands on Mars and rises about 21.9 kilometres above the surrounding plains, although measurements vary depending on the reference level used. Its enormous size is possible partly because Mars has lower gravity than Earth and lacks moving tectonic plates that would carry a volcano away from one location.
Olympus Mons is a shield volcano, meaning it was built by many layers of relatively fluid lava. Its summit contains a broad complex of overlapping calderas rather than one simple crater. The volcano is so wide that its outer slopes are extremely gradual, making it unlike the steep-sided mountains commonly seen on Earth.
Mars also has the nearby Tharsis volcanoes Arsia Mons, Pavonis Mons and Ascraeus Mons. Mauna Kea is sometimes compared with Olympus Mons when measured from its underwater base, but Olympus Mons remains the Solar System’s highest mountain by the usual planetary measurements.