Many of my scenic side trips over the years have included underground adventures. When I was a kid, my family toured some of the caves on the way to or in the Ozarks of Missouri. Meramac Caverns and Bridal Cave are two I remember.
The spectacular Wind Cave National Park in South Dakota was a must see when my husband and I toured the Badlands, Custer State Park, and the rest of South Dakota's attractions.
One of my favorite underground tours was the old Gold Road Mine tour near Oatman, Arizona. I'd never toured a gold mine before, but my imagination used some of the things I learned there to create the fictional mine in The Desert Hedge Murders. Since the Gold Road Mine is now closed, one needs to search other parts of Arizona for a good gold mine tour.
Even though we weren't underground during our Wyoming road trip, we did view amazing sites that had once been buried but were now being excavated by paleontologists. Some of the dig sites at The Wyoming Dinosaur Center are open to tours.
On one of our driving trips in Europe, we discovered we were close to an accessible bunker on the Alpes-Maritimes section of the Maginot Line near the French/Italian border. We were lucky enough to join a group descending into the opening to view the underground stairs, chambers, and tunnels. Even if you don't check out any of the other links, I urge you to take a look at these photos from the Gros ouvrage de Barbonnet near Sospel. I can't even imagine how difficult it was to haul in this equipment and install it below ground. To learn much more about these bunkers and view other photos, check out this 2002 Trip Report from the Bunkertours site.