Mammoth Cave is the world's longest cave at over 360 miles long. With new miles being discovered each year, there are some that estimate the entire length of the cave to be over 700 miles long! So far I've only been on the Frozen Niagara tour. The park has some interesting scenery above-ground as well, including the Green River spring, to many rocks and bluffs and several historic buildings.
Natural Entrance Green River Spring