Descend into a fascinating underground world at Mammoth Cave National Park in Kentucky. With over 360 miles of mapped passages, the park preserves the longest cave system in the world. Each year, more than 2 million people visit Mammoth Cave. Some strolling through large chambers and past unique rock formations while others challenge themselves with a wild cave tour along drop offs and through tight spaces. Photo of a cave entrance by Eric Blankenship (http://ift.tt/18oFfjl).