The astronomy community has been aware of various mountains, valleys, and other geological formations on Mars for the better part of a century, first through observation with telescopes, and later, through spacecraft.

Back in the 19th century, Italian astronomer Giovanni Schiaparelli was first able to discern the vague shapes of the Martian mountain that would eventually be named Olympus Mons. According to Space, scientists would later determine that the giant mountain is actually a volcano.

The mountain is breathtakingly large. Because of the irregular shape of some of its features, it’s difficult to pin down just exactly how far up it rises from the plain below, but by most estimates, it’s in the neighborhood of 16 miles high. By comparison, Mt. Everest is only 5.5 miles tall. Olympus Mons is approximately 374 miles across, and is roughly the same size as Arizona.

Unfortunately, if you were able to see Olympus Mons from the Martian ground, you wouldn’t be particularly impressed. So gentle is its slope that it would appear as merely a slight rise in the elevation of the ground, and it would remain so until you reached the top. Once there, you’d have no idea that you were on the surface of the mountain, as its flat caldera stretches as far as the eye can see in every direction.