When you drive down a mountain what makes your ears experience those uncomfortable sensations?

Your ears pop on a mountain when there's a difference in air pressure between your inner ear and the air outside. The air in your inner ear exchanges pretty slowly. And since the air is much thinner at the top of the mountain than it is near the bottom, if you go down quickly enough (like by driving) the pressures can't equalize gradually. The higher pressure air at the bottom of the mountain presses on your eardrums until they "pop" and the pressure equalizes. That can hurt!