The area around Michigan Tech. attracted many Finnish immigrants. Saunas originated in Finland. They may be more common in houses there...could even be from a previous owner.
Here's this bit from Wikipedia:
Due to the region's geographic and climatic features similar to Finland, the heaviest levels of Finnish Settlement were seen in an area known as the
Finn Hook, which includes northeastern
Minnesota, northern
Wisconsin, and the
Upper Peninsula of Michigan. Today, the region is known as having the highest population of Americans of Finnish Ancestry of any region in the United States; in the northwestern half of the Upper Peninsula of
Michigan they make up the plurality of population.