When we lived in Tennessee, we took our son on a few underground tours, including The Lost Sea Adventure. Different parts of the country offer different types of underground tours. You can see caves, bats, bodies of water, etc.
We’ve also done several underground tours while on vacation. The pictures are from our trip to Turkey last year. My son was 3, and he had fun exploring the underground city with his headlamp.
Have you been underground? What did you see?