Margen
The largest volcano in the solar system is not on Earth, but on Mars. It is called Olympus Mons, and its figures are hard to picture.
It stands about 22 kilometers tall, more than twice as high as Everest, and its base spreads out over hundreds of kilometers, an area comparable to that of an entire country. It is so wide, with slopes so gentle, that if you stood on top of it you would not even notice you were on a mountain: the summit would lie beyond the horizon.
It was able to grow so large because Mars has no shifting tectonic plates like Earth. The volcano stayed fixed over the same hot spot for millions of years, piling up lava without pause.
Source: NASA