As you ascend from the Earth’s surface, the air becomes increasingly less dense. This is because the air at the “bottom” has the air above it pushing down and making it more dense. For more information on the layers of the atmosphere, click here. As air pressure decreases, the air trapped in the inside of your ear will cause your eardrums to push outward, which will make it not only uncomfortable, but also more difficult to hear. The pressure between your inner ear and the atmosphere can be equalized by allowing some air from your inner ear to escape through the Eustachian Tubes. When this happens, you will literally hear the equalization of pressure: the “pop.” A common way to equalize the pressure quickly is to hold your nose, close your mouth, and blow. This works because the lungs have nowhere to push the air put through the ears. This method, however, is most often used by extremely impatient people. When you descend in an airplane, the opposite occurs but it has the same effect: the eardrums will push inward in order to adjust to lower pressure (McWilliams, T., and J. Bass, Dr.).