For centuries mountains have been measured using sea level as the base from where their height is calculated. But the Earth is not perfectly round: it bulges along the equator. And sea level isn"t static, it is pulled and changed by our planet"s gravity. Plus, Everest isn"t sticking out of the ocean, it"s nestled among a landscape of other mountains. Many complex calculations have to be done to establish where sea level would actually be, and Everest"s relative height to it. When that starting point is changed, everything changes.