Most visitors to SDC pass this up, mostly because they don't know it's here. For those that do discover it, the trip is absolutely fantastic. I've been to a few caves in my time, and this is one of my favorites. Once you enter through the cave's ginormous Cathedral room, you'll instantly fall in love with this place.
Each tour takes about an hour, and includes going down 600 steps. Don't worry though, at the end a train takes you back to the park, so you don't have to climb back up. Lantern tours are also available at 5:30 each day, providing a special tour that shows more of the cave, but with only the amount of light that early explorers had to guide you.