The iconic holes in Swiss cheese, like Emmental, are called 'eyes.' For decades, it was believed that these holes were caused by carbon dioxide released by bacteria (*Propionibacterium freudenreichii*) as they consumed lactic acid. However, in 2015, Swiss researchers discovered a more specific cause: microscopic particles of hay dust. During the traditional milking process in barns, tiny flecks of hay would fall into the milk buckets. These particles create 'nucleation sites' where the carbon dioxide gas can collect and expand into large bubbles during the aging process. As modern milking has become cleaner and more automated, the hay dust has disappeared, leading to 'blind' (hole-less) Swiss cheese. Producers now sometimes deliberately add hay dust to ensure the holes remain.