Sitting Bull Crystal Caverns

From website, "Your Underground Adventure Awaits.
Come experience a world like no other. Let our knowledgeable and friendly guides lead you through subterranean, crystalline passages in the Paha Sapa limestone. Marvel at the world’s most extensive display of dog-tooth spar crystal, some as large as 16″ in length. Peer into the still, clear water of Diamond Lake as it reflects the massive crystal ceiling like a giant mirror. Visit the famous French Chandelier room, Crystal Palace, and Rainbow Arch and you will feel as though you are walking through a giant geode. The uniqueness of this experience is sure to create memories you will carry with you for a lifetime. For more information call 605-342-2777."