The "Hole" Situation

At some point while eating Swiss Emmental* (also known as Emmentaler), you may have wondered why are there holes? The holes or "eyes" are caused by a bacteria (Proprionibacterium freudenreichii) that is added in the curdling stages of cheese making. During ripening of the cheese, the bacteria creates carbon dioxide gas. When gas are unable escape, they create the characteristic holes you see in  Emmental cheese.

image via gourmet food world 

*Emmental is a Swiss cow's milk PDO (Protected Designation of Origin) cheese. 

Note:  In the U.S., "Swiss cheese" is a generic term used to describe alpine cheeses or any cheese with holes.