"Imagine a tunnel more than ten storeys underground, a hundred years old, bricklined, wet, and completely inaccessible save by descending through a narrow slit in its ceiling thirty feet above the floor, and then returning up the same rope you came down.
Now imagine that this tunnel flows into Niagara Falls, emerging behind the pummeling curtain of water that nearly everyone in North America journeys to see at some point in their lives."
This tunnel exists! And if you want to see more amazing pics of tunnels, check out The Vanishing Point
I've been down some Cornish mines last year with a sweet local "Mining Nerd" who used to work in these mines before they had to close down in the 80s. That was pretty spooky because it was very narrow at some places and we went down quite deep. What an amazing but terrible job to go down these wet and dirty holes every day, eating Pasties in the dark. Respect to the miners!