Sentence:  There are many such <WSD>mysteria</WSD>: milk is a <WSD>mysterium</WSD> of cheese and butter, and cheese in turn a <WSD>mysterium</WSD> of maggots, which were thought to form spontaneously in rotting food. 
Meaning of the word marked with <WSD> tags:  Any of various unknown elements thought to make up existing forms of matter, or a substance seen as an elemental or pure form of something else.