Know Before You Go
There is a parking area right by the boat put-in for the Harpeth River, which happens to be next to the tunnel. Walk down the stairs and the tunnel will be to your right. If you traverse down to the tunnel and then through it, you will be treated with a beautiful waterfall and swimming hole on the other side. There are other access hiking trails directly to the swimming area that require no dark, dangerous tunnel walking, just follow the signs that say "tunnel access." Be careful and bring a flashlight if you go through the tunnel.