The Vikings got around, including constructing buildings in North America, and even back then, 1000 years ago, wondering who’d built the earlier structures. The Mystery Hill (now called “America’s Stonehenge”) structures are at least 4000 years old, meaning that they weren’t built as “colonial root cellars”, regardless of what they were (much) later used for. :’)
In light of these latest findings, maybe the explanation is that they were trying to find Milwaukee and just took a wrong turn.