Earth’s tallest mountain, Mount Everest, rises to about 5.5 miles above sea level. Mars has it beat by a long way. The Martian volcano Olympus Mons reaches about 13.6 miles tall - roughly three Everests stacked on top of each other.
It’s not just tall, it’s huge. The base of Olympus Mons covers an area roughly the size of Arizona. If you stood at the foot of it, you couldn’t see the summit - it would be over the horizon. The slopes are so gentle that walking up Olympus Mons wouldn’t feel like climbing a mountain. It would feel like a slight upward hike that just kept going. And going. And going.
Mars can grow mountains this big partly because it has weaker gravity than Earth, so massive piles of rock can stand taller without collapsing under their own weight. Also, Mars’s crust doesn’t shift like Earth’s tectonic plates - so a volcano can keep erupting in the same spot for billions of years, building up to extreme heights.
