Of places to see in the United States of America, caves appear high on my list. Because they offer natural shelter (from the elements or predators,) their dark and mysteriousness, and the fact that they are all different, I am attracted to them naturally. Wind Cave National Park in the Black Hills of South Dakota may be the best cave I have seen in my travels.
Essentially, Wind Cave has been created like most caves by the forces of water, limestone and gravity. However, it is different in the shapes of the rock-structures formed over time. Instead of the stalactite or stalagmite forms in caves like Carlsbad Caverns, these structures appear as flower-shaped (rare,) web-like string, and popcorn (see the video below.) As I walked through the Cave, in a constant 58 degrees, amazing displays of colorful and glowing flake and thread caught my eyes. Also, the origin stories of Native Americans coming from the underground into this world through Wind Cave intrigued my mind.
There are walkable, ranger-led tours of Wind Cave a few times daily, but you need to reserve in advance. No bags of any kind are allowed due to the fragile nature of the cave formations. So, (yo Batman!) if caves are your thing, Wind Cave National Park should be on your list.