Soudan 1 was a particle detector located in the Soudan Mine in Northern Minnesota, United States, which operated for a year in 1981–82. It was a 30-ton tracking calorimeter whose primary purpose was to search for proton decay. It set a lower limit on the lifetime of the proton of 1.6×1030 years as well as upper limits on the density of magnetic monopoles. It also served as a prototype for the following Soudan 2 and MINOS experiments.