The tallest volcano in the solar system is on Mars, and it is nearly three times the height of Everest.
Olympus Mons rises about 22 kilometres above the Martian plains and is roughly the size of Poland at its base. Its slopes are so gentle that a person standing on it would not notice they were on a mountain.
Why is this true?
Mars has no plate tectonics, so the crust stays put over a hot spot and lava piles up in the same place for hundreds of millions of years. Lower gravity also lets mountains grow taller before they collapse.
More facts in this category
More about space
Space facts are mostly a lesson in scale. Days longer than years, storms wider than Earth, light that left its star before the pyramids existed — the numbers stop being intuitive very quickly. Every entry here comes from established astronomy and includes a short explanation of what causes the effect, so the fact holds up when someone asks the obvious follow-up question at the dinner table.