Olympus Mons is a very large shield volcano on the planet Mars. The volcano has a height of nearly 22 km as measured by the Mars Orbiter Laser Altimeter (MOLA). Olympus Mons is about two and a half times Mount Everest's height above sea level. It is the largest volcano, the tallest planetary mountain, and the second tallest mountain currently discovered in the Solar System, comparable to Rheasilvia on Vesta. It is the youngest of the large volcanoes on Mars, having formed during Mars's Hesperian Period. It had been known to astronomers since the late 19th century as the albedo feature Nix Olympica. Its mountainous nature was suspected well before space probes confirmed its identity as a mountain.