Mars has not only the deepest valley in our solar system, but also the mightiest mountain. The Martian volcano Olympus Mons reaches a towering height of 27 miles, or three times the height of our own Mount Everest.

This prodigious peak probably formed in the same way that our own volcanoes do: by sitting over a "hot spot" where plumes of hot rock rise up from the planet's interior. But it was able to grow taller than any earthly volcano because Mars lacks plate tectonics, Baker and Ratcliff explain. On Earth, the tectonic plates act like "a conveyor belt over a hot flame," they write. "Volcanoes form, die out, and form anew as the plate moves over the hot spot, producing a long chain of volcanoes." With no moving plates on Mars, Olympus Mons likely sat above a hot, volcano-forming plume for a very long time.