Is Mount Everest the tallest mountain on Earth?

Everest's famous number, 8,849 metres, is its height above sea level, and by that measure nothing beats it. But most of Mauna Kea is underwater. The volcano rises about 4,205 metres above the Pacific and continues roughly another 6,000 metres down to the ocean floor. The US Geological Survey puts the total at about 10,211 metres, well over a kilometre more than Everest.

Then there is Chimborazo, a volcano in Ecuador that reaches a modest 6,263 metres above sea level. Earth is not a perfect sphere. It spins, and the spinning flings the equator outward by about 21 kilometres compared with the poles. Chimborazo sits almost exactly on the equator, riding that bulge, so its summit is farther from the centre of the Earth than Everest's by more than two kilometres.

All three answers are correct. They just answer different questions. If you want the top of the atmosphere's thinnest air, climb Everest. If you want the closest you can stand to outer space, go to Ecuador.