Which mountain on Mars is the tallest volcano in the Solar System?

Olympus Mons on Mars is the tallest volcano in the Solar System.

Olympus Mons rises about 22 kilometres above Mars's reference elevation, making it nearly three times as tall as Mount Everest is above sea level. Its enormous size is partly possible because Mars has lower gravity than Earth and lacks active plate tectonics that would move a volcano away from its magma source.

The volcano is a shield volcano, built by many eruptions of relatively fluid basaltic lava. Its broad flanks extend hundreds of kilometres, giving it a gentle slope despite its extraordinary height. A steep escarpment surrounds parts of its outer edge.

Olympus Mons is located in the Tharsis region of Mars. Scientists infer that it formed over a long period, although its exact age and most recent volcanic activity remain subjects of study. It is not the tallest mountain measured from Earth's sea floor; that comparison involves Mauna Kea.