What Planet Has Olympus Mons?

Olympus mons is on the planet Mars. At a height of about 27 Km it is the tallest known mountain and volcano in the Solar system. It is three times as tall as Mount Everest on planet Earth. It has been suggested by scientists and geologists that the volcano still might have some ongoing activity. It is quite recent in geological terms, estimated to be as old as 1.2 million years. The enormous size of Olympus Mons is because of the reason that planet Mars has not evolved tectonic plates and it exists directly above a planetary hotspot. Some volcanic activity has been observed by Mars Express orbiter's imaging in 2004. It was visible to even the 19th Century astronomers through telescopes.