I've heard a lot about nemo tunnel, but struggled to figure out it's exact location. The route and comments below (as well as the linked resources) represent the best description that I have been able to find thus far.
From Smoky Mountain Jeep Club Forum:
[...] That whole area is so much fun. You can camp, swim, kayak fish. The Jeepin' part is fun too. Finding the road to get down to this area is kinda weird though. Probably even harder to explain via text. Every local in Wartburg knows the Nemo access area though. Once you find your way down the curvy road, sorry don't remember the name. You'll come up on a bridge. Don't go over it if you wanna go through the tunnel. The first dirt trail to the left ( if you are heading down hill toward bridge) is the one you take. Follow that all the way to the train tracks and you can't miss the tunnel. Turn left and there it is! It's a wonderful playground area. Take a trash bag or two because it's sad how many locals left their trash around. I always get a big or two.