Cheese is made when live bacteria is added to milk. Swiss cheese contains 3 types of bacteria, one of which is called Propionibacter shermani. As it reacts with the milk, a fermentation occurs as large amounts of gases are released. In most cheeses, they are able to escape; however, in the extremely dense Swiss cheese, they are unable to break free and form bubbles. These create the distinctive holes found in swiss cheese - known as 'eyes' in the cheese industry.

Did you know that the holes in Swiss cheese are required by the US Department of Agriculture to be between 11/16 and 13/16 inches in diameter? Not sure how Swiss people feel about that. Anyway, now we know why Keith buys so much swiss cheese ;)