Why do your ears pop and ring? --- Daniel, Georgetown, Guyana
Popping and ringing are two different phenomena with completely different causes.
Popping is a mechanical thing. Behind your eardrum you have a semi-sealed air space. There has to be an airspace there or the eardrum would not be able to vibrate. And it has to be semi-sealed because, if the sound waves were hitting both sides of the eardrum equally, the eardrum would be stationary. The problem comes when the air pressure outside the eardrum changes noticeably. If the pressure outside the eardrum drops quickly (e.g. - because you are in a small airplane that goes to 10,000 feet very quickly), the pressure behind the eardrum causes it to bow outward. If the pressure outside the eardrum rises quickly (e.g. - because you swim to the bottom of the deep end in the pool), the pressure causes to eardrum to bow inward.
The eardrum stretches when it bows outward or inward, and this is painful. If the pressure difference gets high enough the eardrum bursts, which is excruciating. So there is a small tube called the eustation tube that is able to let air in and out of the closed space behind the eardrum. When your ears "pop", you are experiencing a sudden change in pressure caused by air flowing through the eustation tube.
[Fun fact - if you have a ruptured eardrum, you can actually blow air through your eustation tube and out your ear as demonstrated here:
A ringing in your ears is caused by the nervous system. When you experience a "ringing" in your ears, which many people perceive as a constant high-pitched tone or constant white noise, it is called tinnitus. There are all kinds of things that can cause tinnitus - everything from taking too much aspirin to listening to your iPod at too loud a volume. See Why do loud noises cause your ears to ring? for details and a nice photograph. See also: