Found This Thing Up North…What is It?

I came across this structure during one of our northern Michigan roadtrips...and I'm not sure what it is. I'm hoping one of you will be able to tell me.

We were driving up between Mackinaw City and Cheboygan when something caught our eyes, hidden by trees. It looked like some sort of obelisk or tower.

Being the curious type, I took the nearest dirt trail and drove down to this thing. At first glance I though it was some old mining furnace...but there is no opening on the side like most of them. So, I climbed to the top and saw a metal trap door - padlocked. So whatever this was used for, it was utilized from the top.

The side had steel-rungs embedded so you could climb it like a ladder, and the top had an old metal railing for safety.

But what is this thing? Just down the road is the Historic Mill Creek Discovery Park so did it possibly have something to do with the area's 1800's lumber production?

Or maybe, since there were wars up on Mackinac Island and Mackinaw City hundreds of years ago, it could possibly have been used to make artillery, such as cannon balls.

Anyway, take a look at the pictures below, see the location, and maybe you can help shed some light on this obelisk for me.

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