National Park Week runs from April 21-29, 2012.

REUTERS

REUTERS

From the longest cave in the world to the longest stretch of undeveloped barrier island, there are innumerable opportunities for exploration within the U.S. national park system that you've may not know about.

Does the name Kobuk Valley ring a bell? How about Isle Royale or Congaree?

There's so much more to the national park system than Yellowstone, Yosemite, and the Great Smokey Mountains -- and there's no better time to get out and explore these hidden treasures than National Park Week, a nine-day fee-free celebration of the 84 million acres of spectacular scenery, historic landmarks, and cultural treasures America has inherited.

National Park Week offers a chance to hike, learn, share, and give back at parks across the nation. In honor of the annual event, here's a look at some of the United States' lesser-known gems.

Kobuk Valley National Park, Alaska

Kobuk

Kobuk Valley National Park, Alaska (wikimedia commons)

Kobuk Valley wins the award for least-visited national park in America. The fact that it's located some 25 to 40 miles above the Arctic Circle may have something to do with it. It's a trek to get to no doubt, but the payoff is undeniable. The Great Kobuk Sand Dunes cover 25 miles of this surreal park where summer temperatures can climb to over 100 degrees Fahrenheit. Kobuk Valley is also a fantastic spot to witness over half a million migrating caribou, whose tracks crisscross the sculpted dunes. While there are no designated roads or trails, visitors can go backcountry camping, hiking, backpacking, and dogsledding. With a park roughly the size of Delaware, however, you'd better know where you're going.

Isle Royale National Park, Michigan

Isle

Isle Royale National Park (creative commons/corvair owner)

Isle Royale offers unparalleled solitude for adventurous backpackers, boaters, and kayakers. Located in northwest Lake Superior, this road-less island park is only accessible by boat or seaplane. Once inside, visitors can explore over 165 miles of forest trails, inland waterways and vast coastline. This wilderness retreat houses a number of species like moose and wolf and gives visitors a chance to observe them in their (relatively) untouched habitat. An oft-forgotten gem, Isle Royale boasts the highest backcountry overnight use per acre of any national park and at the same time is one of the least visited. The park just opened for the season on April 16 and you'll need to make arrangements on a private boat to get in during National Park Week.

Mammoth Cave National Park, Kentucky

Mammoth

Mammoth Cave National Park (creative commons/sturap)

Did you know the United States is home to the world's longest known cave system? Deep below south-central Kentucky lies 390 miles of explored cave passages. This grand, gloomy, and peculiar place with vast chambers and complex labyrinths has earned its name -- Mammoth. To put its size in perspective, the Unesco World Heritage Site is well over twice as long as the second-longest cave system, South Dakota's Jewel Cave. Several cave tours will be free during National Park Week and there will also be special walks and activities for Wildflower Day on April 21 and Junior Ranger Day on April 28.

Padre Island National Seashore, Texas

Padre

Padre Island National Seashore, Texas (REUTERS)

Padre Island National Seashore protects the longest stretch of undeveloped barrier island in the world and is a haven for birdwatchers with around 380 species to enjoy. Rich in natural treasures, Padre Island boasts 70 miles of protected coastline, including a coastal prairie, dynamic dune system, and tidal flats teeming with life. History buffs will enjoy exploring the island's checkered history. Named for Spanish priest Padre Nicolas Brill -- who established the first settlement in 1804 -- Spain, Mexico, and the United States have all laid claim to Padre Island. There will be ranger talks, birding tours, and a stargazing party in honor of National Park Week. Like always, the park has no-reservations camping and plenty of opportunities for fishing, biking, and swimming.

Congaree National Park, South Carolina

Congaree

Congaree National Park, South Carolina (creative commons/hunter-desportes)

Congaree National Park is home to the largest expanse of intact bottomland hardwood forest remaining in the Southeastern United States. With over 25 miles of hiking trails and 2.4 miles of wheelchair-accessible boardwalks, there are plenty of opportunities to spot bobcats, river otter, and wild pigs. This swampy park also offers great opportunities for intrepid kayakers and ranger-led nighttime owl prowls for kids.

Canyon De Chelly National Monument, Arizona

Canyon

Canyon De Chelly National Monument, Arizona (creative commons/Don & Suzan)

For nearly 5,000 years, people have lived uninterrupted near Canyon De Chelly -- longer than anywhere else on the Colorado Plateau. Today, the National Park Service and Navajo Nation work together to manage park resources and preserve both the landscape and a dying culture. A handful of Navajo families still live within the park borders, raising livestock and farming the land. There will be free ranger-led hikes and programs for National Park Week and camping and Navajo guides will also be available for a fee.

Wind Cave National Park, South Dakota

Wind

Wind Cave National Park, South Dakota (wikimedia commons)

Though it's no Mammoth, Wind Cave is the fifth-longest cave in the world -- and explorers are still finding new passages and rooms today. The 135 miles discovered so far provide endless opportunities to see unique formations including the world's most outstanding example of boxwork, a honeycomb-patterned mineral structure formed by erosion rather than accretion. Besides the cave, the park is also home to over 28,000 acres of prairies, forests and wilderness. The mixed-grass prairie is one of the few remaining in the U.S. and is home to bison, elk, pronghorn, mule deer, coyotes, and prairie dogs.

Wrangell-St. Elias National Park and Preserve, Alaska

Wrangell–St.

Wrangell–St. Elias National Park and Preserve, Alaska (REUTERS)

Wrangell-St. Elias is larger in size than the country of Switzerland, stretching from one of the tallest peaks in North America, Mount St. Elias (18,008), to the Gulf of Alaska. The largest national park in the country houses nine of the 16 highest mountains in North America. Over 13 million acres of hiking, fishing, camping, rafting (and generally surviving) make this park one of the most exciting places on the planet. The two (mostly unpaved) roads of the park allow adventurers to see much of the interior wilderness. If you don't want to drive through uncertain weather and road conditions, you can take scenic plane ride to truly appreciate the vastness of the massive reserve. Depending on the season, you can expect to encounter caribou, moose, grizzly and black bears, mountain goats, gray wolves, coyotes, red foxes, and wolverines.