Why Do Our Ears Pop When We Go Into a Tunnel?

Inside our ear is a little tight piece of skin called the eardrum. Usually the air inside the ear presses as hard against the eardrum as the air outside. But in a tunnel the air inside the ear presses harder, and that makes the ear pop. The eardrum is a very thin membrane, only .004 of an inch (0.1 mm) thick. In normal circumstances the air pressure on it from the middle ear is about the same as pressure from the outside. When the outside air pressure is suddenly reduce-as when entering a tunnel-the narrowness of the ear passage does not allow the air pressure in the inner ear to adjust at once, and our ears are uncomfortable until it does. When our ears pop we can get them back to normal by swallowing or opening our mouth and yawning.