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.

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.

The breathtaking beauty of Alaska sometimes hides the fact its winters can be incredibly harsh, especially for those who live in the state's outlying areas. "Alaska: The Last Frontier" perfectly illustrates this reality, as the series profiles life for the Kilcher family in the isolated community of Homer. For four generations the Kilchers have lived off what their 600-acre homestead has provided, but cultivating that living is never easy. Led by patriarch Atz Kilcher and his brother Otto, the family spends the short summer and fall gardening, hunting and fishing for food, gathering supplies from the land and preparing their animals for the winter. Viewers, who may or may not have a fancy phone by their side while watching on their big-screen high-def TV, also see the Kilchers living off the grid, where running water and electricity aren't daily staples, nor is contact with the outside world. Atz, by the way, is the father of music superstar Jewel.

TV's most extreme survival challenge just grew extra-large. Veteran survivalists familiar with the stripped-down drill after appearing on "Naked and Afraid" attempt to survive in a desolate, dangerous environment for 40 days -- 19 more than they previously experienced on the show. Pushing the very limits of human endurance, the men and women must all vie for the same limited food, water and shelter while avoiding territorial-sensitive predators and venom-filled reptiles. Because there is no other choice, the competitors quickly get to know one another -- and their surroundings -- and hope that their instincts, survival skills and intestinal fortitude serve them well.

Discovery Channel takes the ubiquitous survival show theme to the next level by stripping it to its bare essentials. In "Naked and Afraid," complete strangers -- usually a man and a woman -- meet in a very unique way: They're stranded in a dangerous, desolate location, without food or water, and they're completely naked. Each episode follows the adventurers as they attempt to survive on their own with nothing but a personal item and the knowledge that the only prize is their pride and sense of accomplishment. Because there is no other choice, competitors quickly get to know one another -- and their surroundings -- and hope that their instincts, survival skills and intestinal fortitude serve them well.

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.

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.

Surviving in the wild is not an exact science. Beyond the basics of finding water, food, shelter and eventually help, opinions vary. Two experts tell how they think it's done in this series, as initial episodes feature trained survivalists Dave Canterbury and Cody Lundin demonstrating the skills and creative thinking that can help one cope in tough surroundings. Equipped with minimal gear, the pair are left in remote locations -- an uninhabited island off Nova Scotia, a New Zealand mountaintop, deep in the Everglades, the jungle of Thailand, and the desert of Baja California, to name a few -- where they battle the elements and sometimes each other. Season 3 introduces U.S. Special Ops veteran Joseph Teti to replace Canterbury, in the middle of Season 4 primitive survivalist Matt Graham of Discovery's "Dude, You're Screwed" replaces Lundin, and partway through Season 5 producers forge new partnerships by rotating survival experts as hosts.

Money and goods mean nothing to the people in "Live Free or Die." The series depicts a trend called "rewilding" -- the undomestication of humans -- and follows those who've rejected a mainstream existence to live off the land, in simple homes without electricity or running water. Being self-sufficient is a constant challenge, as obstacles like brutal weather and depleted food stocks require quick, innovative solutions. Modern pioneers include Colbert, a former financial adviser now living in a Georgia swamp; Gabriel, whose California lifestyle alternates between the mountains and the sea; and Tony and Amelia, who turned a hillside in the Blue Ridge Mountains into a garden.

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.