Answer: Mars

If you want to see the tallest planetary mountain in the solar system, you’d better pack your bags because you sure aren’t going to see it from the comfort of Earth’s familiar atmosphere. The tallest planetary mountain in the solar system is located in the western hemisphere of Mars and rises an astounding 13 miles (21 kilometers) above the Martian sea level (if there were still seas to speak of) and 13.6 miles (21.9 kilometers) off the now-dry sea beds.

Even without the extra elevation of the dry sea bed factored in, Olympus Mons is still two and a half times as tall as Earth’s Mount Everest and taller than the next tallest mountain on Mars by a few miles.

Curiously, the tallest mountain in the solar system isn’t even on a principal planet, but located on the enormous minor-planet/asteroid 4 Vesta. 4 Vesta is 326 miles (525 kilometers) in diameter and sports a mountain, Rheasilvia Mons, that is 14 miles (22.5 kilometers) high.

While the mountain on 4 Vesta is the tallest in terms of raw height, the scale of it compared to the body it is on is truly impressive. If Mount Everest was as tall as Rheasilvia Mons (relatively speaking), it would tower around 340 miles (547 kilometers) above sea level.