Mammoth Cave was the first American national park to visit, which featured a trip to the underground cave. The landscape in the cave is changing, the open place can be a conference hall, the narrow place is like a line of sky, there are flat and twists, the flower-like crystals when entering the hole make people linger, the stalactites known as Niagara Falls when leaving the hole are breathtaking, The feeling of satisfaction came alive and wanted to come again.