This is the longest cave system in the world with tunnels of more than 400 mi and still counting. National park services offers some interesting tours, some 5 hrs long exploring deeper sides of the cave. However, you need to make reservations ahead of time if you plan to visit during a holiday or weekend. We failed, so we only saw a small portion of the cave on two separate tours. We spent some time exploring the surface.
1. Historic tour. There are no typical cave formations in this part of the cave. It’s just a large tunnel and they talk about the mining past of the cave.
2. Drips and Domes tour. This was a prettier side of the cave with stalactites and stalagmites, but we had share it with another 100 people or so.