Tunnels as a general rule don't make me too nervous. But this one in Custer State Park (South Dakota) caused me to fret a bit. It's only one lane, and both directions must use it. Aren't those trees cool just clinging to the sides of the rock?
About halfway through the top opens up and it seems you're just in a crevice.
There's an area to park once you get through and we paused to look back the way we had come.