11. Isle Royale National Park, Michigan
To begin with, there are no roads leading to this park and one has to go through a boat or a sea plane. Not even bicycles are allowed. Then, the Rock Harbor Lodge is the only available place to stay but camping is allowed and you can backpack and have loads of fun here. One of the best sights here are the ships littered at the bottom of the lake visible through the clear waters. The Isle Royale is a haven for scuba divers and the quiet bays offer kayakers a great time.
12. Mammoth Cave National Park, Kentucky
The end of this cave has not even been found yet, which makes it mammoth, right? It is a five-level labyrinth that has quite the geological triumphs. Over 5,000 years ago, there were Indians here who used mussel shells to shave gypsums off the wall. In addition to the underground, the entire above ground offer trails that wander beneath great oaks. The green River flows on top while the River Styx Spring Trail sprouts from the cave entering the Green River.