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POPSElephant Rocks State Park, MO. There is a really good picture of the whole circus train from below on the side of the hill at wikipedia, http://en.wikipedia.org/wiki/Image:Elephant_Rocks_SP_1.jpg titled the largest of the Park's elephants, named Dumbo, leads a train at the top of the hill
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POPSCrystal Cave According to the site, the crystals formed while the cave was filled with liquid. Its 125 - 150 degrees and 100% humidity in there, so the scientists are having a hard time documenting it. The mineral is gypsum, go to site for more pictures.