Duluth Minnesota was built over a swamp. In the early 1900's as the city grew the rivers running through the valuable land were routed through tunnels to the lake.
there are several of these tunnels and most are in their original 1900's stone masonry condition
This was the purest darkness I have ever experienced. The complete absence of light was an interesting experience but made photography challenging.
On this trip we started on the lake end of the tunnel, the farther up stream you travel the older and smaller the tunnel gets.
On a steep downhill they used large slabs to make stairs, a hundred years of water flow has drastically carved these away
This is my first post, sorry if I got anything wrong.