4. Mammoth Cave National Park – Kentucky
The name says it all. With 400 miles of explored passage ways, Mammoth Cave is the longest cave system in the world.
Renowned for its size and vast network of extremely large horizontal passages and vertical shafts, a trip through the cave reveals impressive stalagmites and stalactites, beautiful gypsum flowers, delicate gypsum needles, and over 130 species of flora and fauna.
Explore the cave with a short self-guided (free), or book a longer guided tour. Choose from a variety of tours including the six-hour Wild Cave Tour ($55 ages 16 and up) which takes you on through five miles of less developed parts of the cave system.
For a shorter, less challenging experience try out the 1.5 hour Frozen Niagara Tour ($13 adults, $9 youth), or the two hour Domes & Dripstones Tour ($15 adults, $10 youth), which both offer views of dramatic formations and relevant commentary.
Be sure to book your tour in advance as this cave is very popular and tours sell out daily.