Olympus Mons is a large shield volcano on Mars. As measured by the Mars Orbiter Laser Altimeter (MOLA), it is 21.287 kilometres (69,840 ft) high, about 2.5 times the elevation of Mount Everest above sea level. It is Mars's tallest volcano, its tallest planetary mountain, and is approximately tied with Rheasilvia on Vesta as the tallest mountain currently discovered in the Solar System. It is associated with the volcanic region of Tharsis Montes. It last erupted 25 million years ago.