Wind Cave National Park!  Boxwork is the unique formation found only in Wind and Jewel Caves in South Dakota.  Since it is found nowhere else in the world,  that makes this a pretty unique place to see.  The boxwork is the result of a chemical reaction between CO2 and O2.  The gist is that it would be like building a house out of sugar cubes and mortar.  When the sugar cubes dissolved in rain you’d be left with the interesting mortar structure.  That is boxwork and it was everywhere throughout the cave. Amazing!