Why does cheese have holes?

Germany

The holes are made by sizable bubbles of gas produced by a special kind of bacteria (propionic) in the cheese-ripening process. While the cheese is ripening (maturing), the long molecules of the cheese break down into simpler, tastier molecules by the action of enzymes. Parallely, propionic bacteria (CO2- forming bacteria) will produce carbonic gases and contribute to the creation of a texture with round holes/eyes or irregularly shaped eyes. Some cheeses have extra large holes due to the addition of propionic acid which also gives the sweet, nutty flavour.