Michigan E Shaft
The Minesota Mine (yes, spelled that way) was one of the first wildly rich copper mines in the Copper Country. It closed, in the words of the copper handbook, largely due to management unwilling to invest the necessary money in keeping it going. It was reborn as the Michigan Mine, on a much grander scale.
When it began to play out (again), they expanded and drove new shafts, including the "E" shaft. Its fortress-like ruins still sit atop a short bluff, with its collapsing shaft recently re-capped. I have no clue what happened to these ruins to cause them to fall apart this way -- it's very unusual, even in the ruin-ridden Copper Country.