Jewel Cave National Monument, at 151 miles, is the second longest cave in the world. The longest cave is Mammoth Cave in Kentucky. Jewel Cave gets its name from the abundance of calcite crystals that completely cover the cave walls. Calcite is the same mineral that they use in making soda. The calcite is also the mineral that eats away and dissolves all the limestone to make the cave. The on going joke that all the tour guides say is that the calcite makes Jewel Cave "Nature's Cavity". The cave tour was nice. Our tour guide, a college girl, had a huge crush on two of the early 20's guys that were there on vacation. She would ignore everyone else in the tour but answer all their questions and walk with them. They didn't seem to interested in her however.