In analyzing satellite images between 1998 and 2012, researchers at the Michigan Tech Research Institute were surprised to find that lakes Michigan and Huron are now clearer than Lake Superior. In a study published late last year, the researchers say limiting the amount of agricultural and sewage runoff in the lake has had an immense impact. However, the emergence of invasive mussels, which number in the trillions and have the ability to filter the entire volume of Lake Michigan in four to six days, has had an even greater effect.