The crystal caves in Mexico have to be the most beautiful and amazing place on earth. Look at how gigantic some of these crystals are… up to 11 meters (36 feet) long! Made of gypsum, the crytals are also extremely slow-growing and have taken millions of years to form underground in a hot toxic liquid. The caves were pumped out so that explorers could enter them.
http://news.nationalgeographic.com/news/2010/10/101007-lost-crystal-caves-mexico-science-mine-superman-ice-palace/