Mammoth Cave is located in the United States. It is one of the A ___ cave systems in the world. Visitors who go there B ___ explore many underground passages and unique rock formations. While I C ___ the cave, I saw stunning stalactites and stalagmites. You must stay on the marked trails because some areas are fragile. There D ___ a lot of interesting facts about the cave's history. Everybody who E ___ adventure should visit this amazing place.