Alaska's unique wilderness race, the Susitna 100, challenges athletes on bikes, skis and running shoes to brave the elements and test their own limits. David Allan/CNN
At dawn, an announcer called the racers to the starting line with bikes in front, and counted down to the start. The starting line is about a 60 mile drive from Anchorage. Courtesy of Susitna 100
Racers begin down an icy road that turns into a trail that becomes wilderness within the first mile. Soon after that, the pack starts to thin out as bikers pull ahead.
The race course winds through small frozen lakes, rivers, bogs and woods in the shadow of the Chugach Mountains. David Allan/CNN
There is no cap on the number of racers; Alaska has room for everyone. This year, 101 bikers, 31 runners and 15 skiers registered.
Most runners pull a minimum of 15 pounds of required gear behind them in a sled. The mandatory gear includes a sleeping bag rated minus-20 degrees Fahrenheit, a bivy sack or tent, sleeping pad, and a day's worth of food. David Allan/CNN
The race is open to everyone, but is not exactly for everyone. You have to be very fit, equipped for mental highs and lows, nimbly prepared for varying race conditions and brave. Courtesy of Andy Romang
Ultra marathoners compete in the 100-mile run nearly continuously for 18 to 48 hours. Before it's over, some runners will have experienced three sunrises, two nights of running, perhaps a few harmless hallucinations, serious blisters and numbing sleep deprivation.
Specialized fat-tire bikes are built for such conditions. People can complete the beautiful but chilling course on a bike in anywhere from eight to 40 hours. CNN
The dangers of injury, frostbite and hypothermia are rare, but quite real. CNN
Conditions vary so much from year to year, athletes are allowed to change their mode of transportation up to the last moment. David Allan/CNN
During the day, the view is 50 shades of white as snow covers the lakes, black spruce trees and the trail. Evelio Contreras/CNN
About 10% of the racers don't finish, usually voluntarily. For some, it's about the race conditions, others get hurt or are feeling ill. Some just find it too demanding, either because they didn't train properly or didn't have the right gear.
The finish line is the same place where the race started, the Happy Trails Kennels, owned by Martin Buser, a winner of the famous Iditarod dog sled race. Courtesy of Susitna 100
The finish line is a rather quiet affair. Some racers may have a friend or loved one there to cheer them on, but finish times are too spread out and the temperature is too cold to have groups of people standing around waiting for hours. Courtesy of Andy Romang
There is no prize money for the category winners, just bragging rights. All the finishers get a T-shirt and belt buckle and an enormous sense of accomplishment. Courtesy of Andy Romang