Pop Quiz: What’s the highest mountain? Answer? It depends. Oh, okay, the answer on Earth is Mount Everest — standing an amazing five-and-a-half miles high. Pretty impressive — unless you live on Mars, where there is a mountain called Olympic Mons that stands fifteen miles high. So why is Olympic Mons so much bigger than Everest? Another way to ask this question is: Why aren’t there any mountains on Earth fifteen miles high like Olympic Mons? How about thirty miles high? Or a thousand?