The eight explorers are on the fourth leg of their expedition and are struggling to stay afloat on the Talachulitna River.

Helicopters leave the eight explorers on a snow-capped summit where they must descend a ravine and then traverse eight miles of the Triumvirate Glacier.

The men are divided into two groups and must build two competing rafts; one weighs in at nearly 2 tons while the other group hopes their small raft will be able to maneuver in the turbulent Yukon.

Some take the higher land, some follow the river as the men choose their route, determined to be the first to arrive.

Competitors brace for one last battle with treacherous glaciers, rivers and mountains; teams are dropped in a glacial river valley and must reach the insertion flag where to find a cache of mountain bikes.

Only few wins remain with just two legs to go; teams must design and build handmade rafts to survive the roughest whitewater they have faced yet.

The elevation takes its toll in leg eleven as teams climb a steep Alaskan peak to reach the insertion flag, descend 2,000 feet to the Triumvirate Glacier and battle 24 miles of frigid terrain to reach Beluga Lake.

In leg ten, teams are dropped on a remote beach and can travel by land or by sea; facing dehydration, a giant pod of whales, sickness and violent rapids.

Teams are dropped on Kodiak Island and must face fierce Kodiak brown bears; teams take different routes; one man becomes separated from his team and comes face-to-face with a massive predator.

Teams are dropped on the banks of the Tazlina River and must build log rafts to conquer punishing rocks and class four whitewater.

Beginning leg seven on a remote tundra near Mount McKinley, known for some of the most brutal terrain in Alaska.

During leg six, teams must slash through a thorny gantlet of devil's club, then some head for high ground while others bushwhack through the forest.

Teams ride the historic Alaskan Railroad to a remote river valley and must traverse mountains, canyons, glaciers and an iceberg-filled lake to reach extraction.

On leg four, the teams journey by land and water as they race toward the extraction LZ on an ocean inlet; The adventurers must choose to either take boats down Six Mile Creek or make a grueling hike over a mountain pass.

Teams compete in leg three; descending 2,000 feet of snowy ridges, slopes and swamps to reach the landing zone at Talachulitna Lake.

In the second leg of the competition teams are dropped near the top of a mountain peak and must descend the mountain in any way possible.

In the first leg, teams must cross a rapidly thawing lake, crevasse-filled glacier and walls of crumbling ice to reach the summit of Mount Gerdine.

The three remaining teams battle to the top of Mount Augustine, for the glory of being crowned champion.

The teams face their toughest challenge yet; a race across an icy glacier, down a white-water river and across the open ocean.

The three remaining teams go to Kodiak Island, a temperate rainforest full of hunting opportunities.

The teams head out on an island ruled by Kodiak bears, one of the largest bears on Earth.

When bears, wolves and foxes are your only neighbors, life can be pretty lonely. Add minus-60-degree days and a constant battle for the most basic necessities, and you have the daily challenges of people who live in remote corners of Alaska. This series takes viewers deep into an Alaskan winter to meet tough, resilient residents as they try to stay one step ahead of storms and man-eating beasts to survive the season. The closest neighbor to Sue Aikens is more than 300 miles away. Eric Salitan subsists solely on what he hunts and forages. Chip and Agnes Hailstone catch fish for currency in bartering for supplies, and Andy and Kate Bassich use their pack of sled dogs for transportation. Also highlighted is a time of year not always part of what viewers see in Alaska: spring! Ice is breaking, animals are waking, and residents face new tests before deep cold returns.

Each year adventurers make Alaska's mighty Yukon River their home for five weeks. They float downstream on homemade log rafts to sell firewood and other supplies to remote villages. The reality-documentary "Yukon River Run" presents hourlong episodes tracking the progress of various crews. As harsh winter months approach and threaten both safety and success, stakes are heightened as rafters endeavor to cash out and escape from the cold.

Patrolling America's largest state is the job of roughly 400 troopers in one of the toughest law enforcement agencies in the nation. Essentially, these cops say, nearly every Alaskan resident is armed and they know how to use their weapons, which makes any scenario a trooper encounters a potentially fatal one. Follow along as the "blue shirt" Alaskan State Troopers police the towns and villages, and the "brown shirt" Alaska Wildlife Troopers enforce regulations covering both commercial and sport fishing and hunting activities.

Wilderness guide and survival instructor Hazen Audel attempts to stay alive in some of the world's most inhospitable places by using centuries-old techniques. Hazen joins tribes in the rainforests of Ecuador, the Kalahari Desert of Namibia, the mountains of western Mongolia, the frozen Arctic of Canada, in equatorial Kenya, and on a remote island in the Pacific Ocean to learn the skills firsthand. He hunts with bows and poisoned arrows, climbs harsh mountains in search of prey, and harvests food beneath shifting sea ice, among other challenges that push him to his limits.

A seasoned adventurer and survivalist deliberately "strands" himself in remote locales and makes his way back to civilization to provide in-depth advice for travelers who may find themselves lost on what was expected to be a routine hike or other trip.

The old Dolly Parton hit "9 to 5" isn't a tune worth humming for the blue-collar pioneers featured in "Filthy Riches." The series spotlights ingenious Americans who skirt a conventional workplace in favor of making a living in the deep rivers, soggy mud flats and wild backwoods of the U.S. Ray Turner, for example, has been catching eels in Delaware for 30 years. He uses a self-made smokehouse in the woods to cook the critters and sell them. Billy Taylor and his sons hunt for prized ginseng root in the Appalachians. Taylor, a fully licensed wild ginseng dealer, promotes sustainability by planting its berries. In Maine, Jim Campbell and Andy Johns make the coastal mud flats their office, as they dig for valuable bloodworms to sell to fishermen. And Greg Dahl and Albert DeSilva are burl hunters. A burl is a hard, unwieldy outgrowth on a tree, usually at the trunk. Burls have value because of the spectacular patterns found in them when cut open.

Famed survivalist Bear Grylls says goodbye to Hollywood and takes celebrities on journeys into some of the wildest locations in the United States and around the world. Whether it's searching for food in the most remote environments or battling intense weather, each episode chronicles the celebrity's experience of pushing their body and mind to the limit to successfully complete the adventure.

Mick Dodge is one with the woods, having left modern conveniences behind 25 years ago to live among the trees, caves and animals in Washington state's Hoh Rain Forest. It's not an easy life by any means -- he sleeps in tree stumps and has no easy access to food -- but each day presents a different adventure, and as the always-barefoot Dodge says, "All I have to do is follow my feet." He's walking in the footsteps of four generations of Dodge men who have called the Olympic Peninsula their home, and because the intensely private former Marine allowed National Geographic access to his world, this time viewers are welcomed to witness the primal life of "The Forrest Gump of Middle Earth."