Why do your ears pop on a plane or up a mountain?

Right now, a column of air stretching from your head up to the edge of space is pressing down on you, many tonnes of it, far heavier than a car, spread over your whole body. You never notice it, because it pushes from every side at once. But take a lift up a tall building, drive up a mountain, or climb into a plane, and suddenly your ears go strange and muffled, then *pop*. Nothing touched them. So what just shoved your eardrums around?