The Underground Booty Squad
Minneapolis is known for its grain milling history. Just south of Nicollet Island is a large waterfall, St. Anthony Falls, which at one time powered numerous grain mills on both sides of the river. "Headraces" and "tailraces," tunnels that carried water to and from the mills, were dug, connecting the shafts and wheels powering the mills to both upstream and downstream of the waterfall. Little remains of the headraces, but an extensive system of tailraces can be entered on both sides of the river.
The west side tailraces are connected in a haphazard, more or less tree-branch pattern. This area of the river is being converted to a park to allow tourists to explore the tailraces (hopefully without a guide!!!) and all entrances have been secured with metal gates. At the time that I explored these tailraces, the water was deep enough that you need an inflatable boat to cross the first tunnel back from the entrance, and the rest of the system was filled with 1-2' of pure mud. Many rusty shafts about 2' in diameter lead up from these tunnels to the ancient mill sites. There are many random crawl tunnels, almost as wide as the main tunnels, which lead to mysterious crawl spaces that go on as far as one is willing to follow them. At the end of one tunnel, an ancient metal spiral staircase led back outside.