Mark believes that the U.S. is under threat of foreign occupation; Joe prepares for a cyber attack and builds a post-apocalyptic schoolhouse.

Two Tennesee families prepare for an earthquake in the New Madrid Seismic Zone; David Mays is building the ultimate doomsday drone fleet. While David Nash is building a geodesic dome; one of the most stable structures on earth.

Near Fort Worth, Texas, Vietnam veteran Richard Huggins is preparing for a nuclear strike by a terrorist state. Meanwhile, in rural Colorado, Dr. Dave is prepping for a possible solar flare.

Curt and his family have designed a fortress deep in Oregon's back country in the event of an economic collapse.

Chad believes that a nuclear strike resulting in a genocidal siege is a real possibility. He is working on executing an escape tunnel from his family's house to a vehicle.

Kenny and his wife are preparing for a civil war with a fortified bulletproof rooftop command center; Kevin is prepping for an EMP attack by building an underground bunker.

Two families who fear the end of the world is coming are preparing for Category 5 hurricanes and a complete economic collapse.

Rob is devising the ultimate booby-trap system. Greg is preparing for a financial catastrophe by building an invisible tree house where he hopes to be safe from looters.

Tracy prepares for a devastating tornado that can throw buildings into the air and hurt hundreds of people.

Marine Alex Dunbar trains a private army of German Shepherd attack dogs in San Luis, Colo.; Brenda McSwigan fears that a vicious mutating virus will lead to a pandemic.

An economic collapse could mean total chaos. In Washington state, Steve works with a stern hand to prep his family for the potential threat. South Carolinian David Appleton is a comedian, but the idea of a devastating earthquake is no joke to him.

A couple prepare for a day when the north and south poles swap places; a man in California demonstrates how he plans to survive in an urban jungle in the event of a massive earthquake.

Some preppers like Jeff Mann believe in safety in numbers, while others like Tony, who believe that an end of days asteroid will turn the Earth into ash, are independent.

Former firefighter medics move to the mountains in preparation for a total economic collapse; a man who owns a construction company has built a bunker; a man prepares for a terrorist attack.

Fearing a huge electromagnetic pulse, Brent has built a castle to protect his family. Derek's family uses their Wild West amusement park to practice strategic defense.

Brian smith prepares for collapse of U.S. monetary system and fears the world's food supply will dry up.

A woman who began prepping after being warned of danger in her sleep; a couple prepare for a collapse of the world agricultural system by planting hundreds of edible plants; a man who fears a cyberattack on the power grid.

The Coy family live in the shadow of Mount St. Helens; Bill Simpson has spent six years building a sailboat bunker to protect his family from electromagnetic pulses and any chaos that will occur on land.

Kevin and his family decide to move to Costa Rica because they believe an economic collapse will make America unlivable.

A man prepares his family for an electromagnetic pulse attack; a New York firefighter prepares for the eruption of the Yellowstone super volcano.

Mike and his girlfriend Freda fear that the current political tensions and economic unrest will lead to a world war; Mike Adams is anticipating a terrorist attack; Joe, has given up on modern living.

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.

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.

Extreme survivalists go head to head in a race across the Alaska wild. Using ingenuity, experience and just the gear they can carry in their packs, the participants have 60 hours to reach the finish point of each leg of the adventure -- the series features 13 legs -- and in addition to navigating treacherous glaciated river valleys, barren ridgelines, and high mountain peaks, the challengers battle hunger, dangerous predators and unpredictable weather. There is no grand prize awaiting each leg's winner, other than the pride of accomplishing a grueling feat. For season three, the 12 competitors are divided equally into four teams -- Military, Endurance, Alaskans and Lower 48.

Cameras follow the action along Arizona's Sonoran Desert, one of the busiest border crossings in the country, as officers and agents of U.S. Customs and Border Protection fight terrorism, apprehend drug smugglers and intercept people entering the United States illegally. The series also embeds with Customs and Border Protection, and with local law enforcement, as units patrol near and on the border in South Texas.

Meeting people from all cultures whose lives are shaped by different fundamental forces.

Academy Award winner Morgan Freeman explores the meaning of life, God, and many big questions in between in an effort to understand how religion has evolved and shaped society. A different divine subject is covered in each hourlong episode, titles of which include "Creation," "The Devil Inside," "Afterlife," "Apocalypse," and "Who Is God?" To explore these topics, host and narrator Freeman visits nearly 20 cities in seven countries to see some of the world's greatest religious sites, among them Jerusalem's Wailing Wall, India's Bodhi Tree, Mayan temples in Guatemala, and the pyramids of Egypt, and he immerses himself in religious experiences and rituals. "In some places I found answers, and others led to more questions. The constant through it all is that we're all looking to be part of something bigger than us. If there's one thing I've learned, it's that we certainly are," Freeman says.

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."

Advanced computer graphics, forensic science, eyewitness accounts, interviews with experts, archival footage and re-enactments piece together in great detail the events that led to some of the biggest disasters of modern time.

Selling real estate in the Los Angeles area can be a glamorous job, as "Million Dollar Listing" proves. The show follows young real estate agents as they try to sell high-end properties in the Los Angeles area, including Beverly Hills and Malibu. The agents use their charm, good looks and business savvy to stay on top of the competitive heap in the real estate market. Will a Los Angeles housing slump make it more difficult for the agents to sell their properties?

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.