What is the tallest mountain in the world?

The answer depends on how you measure, and there are three legitimate champions. By the standard measure, height above sea level, Mount Everest wins decisively at 29,032 feet (8,849 meters), and that is the answer for practically all purposes and the reason Everest is the ultimate mountaineering prize. But Everest is only the highest, not necessarily the tallest, and the distinction matters. Measured base to peak, the total height of the mountain itself, Mauna Kea, a volcano in Hawaii, is actually taller: it rises about 33,500 feet from its base, but most of that is underwater, so only about 13,800 feet pokes above the sea. From foot to summit it is a bigger mountain than Everest; it just starts far below the ocean surface, so it loses the sea-level contest.

The third champion is the strangest and depends on measuring from the center of the Earth. Because our planet is not a perfect sphere but bulges outward at the equator (it spins, and the spin flings the middle out), points near the equator are farther from Earth's center than points near the poles. Mount Chimborazo in Ecuador sits almost on the equator, and although it is far shorter than Everest above sea level, its summit is the farthest point on Earth's surface from the planet's core, beating Everest by over a mile on that measure. So the honest answer: Everest is the highest above sea level (the usual meaning and the real climbing challenge), Mauna Kea is the tallest base to peak, and Chimborazo is the farthest from Earth's center. All three are correct answers to slightly different questions, which is exactly why this makes such a good argument-settler.