Frostbite, sleep-deprivation and harrowing descents in pitch blackness are all part of the strange allure of the Arrowhead 135, a two-day 135 mile (217 kilometer) race that takes place each winter in far northern Minnesota.
The competitors ski, bike or run along a snowmobile trail from the Canadian border, through a remote stretch of forest, to the small town of Tower, Minnesota. One hundred and thirty-five athletes started this year's eighth annual Arrowhead 135 in late January.
Last year, overnight temperatures plummeted to minus 31 degress Celsius. The year before, minus 37. That year, cyclist Jason Buffington saw one of his friends - a fellow racer - who'd stopped.
"I came up on Charlie in the last 20 miles of the trail standing and waving his legs back and forth trying to get circulation back in his toes," Buffington says.
Charlie Farrow, 52, kept going, crossing the finish line about two hours later. Buffington, a doctor from Duluth, Minnesota, and 10 years younger than Farrow, was there to meet him, and quickly helped him remove his boots.
"His toes were swollen and purple like a plum," Buffington remembers. "He lost probably about half of the skin off his big toe about two or three months later."
"My toenail never came back," Farrow adds, "so I'm a man without a toenail."
Nevertheless, both men were still at the starting line for this year's event. Farrow's bike was outfitted with snow tires as wide as his fist. After biking the past two years, Buffington raced on foot this time. He rigged up a sled to pull behind him, loaded with more than 11 kilos worth of survival gear every racer is required to carry for the extreme cold.
"You get what's called the kennel cough," Buffington says. "Where your lungs get frozen, your eyeballs, your corneas get a little frostbite, and everyone kind of walks around, and everything's real foggy, and you just have this dry coughing going the whole time."
Then there's the lack of sleep. The walkers and skiers take almost two days to complete the course, and may only sleep a couple of hours. The fastest bikers take nearly 20 hours, and don't rest at all.
This was Farrow's seventh Arrowhead. He's done all of them except the very first race. The high school social studies teacher says every time he does the race, his fatigued mind starts playing sinister tricks.
"I have a recurring hallucination regarding the Wizard of Oz. I always have this vision of the trees coming after me... and then I also have this vision of the Emerald City... but I can't ever get to it," he says.
Isolation is also a factor. As the course meanders from International Falls, on the Canadian border, through a national forest and around and over some of Minnesota's 10,000 now-frozen lakes, the racers are spread out far apart.
"That's definitely the biggest danger," Buffington says. "Both years that I've biked it, even though it's taken less than 20 hours, there are times where for six-and-a-half hours, in the middle of the night, 20 below, you don't see a soul, and if anything happens, you're out there on your own."
That means racers have to be extremely prepared and careful. Three aid stations and nine shelters are spaced along the route.
Jeremy Kershaw, 40, a cardiac nurse, has completed the race for the past three years; first by ski, then bike, and last year on foot. With about 32 kilometers to go, he caught up to a racer struggling on the side of the trail.
"He was kind of frantically trying to get new clothes on and eat," Kershaw says. "It was a scary situation because I was really at the last several hours of the race, and so I was really at the end of my reserves."
Luckily, the racer had a cell phone, so Kershaw was able to get hold of a support crew which hauled the racer to safety by snowmobile.
Kershaw says it was a good reminder of how things can go wrong. "If you're not paying attention, things can go south very quickly, particularly when it's that cold and you're that tired."
Every year, about half the racers in the Arrowhead 135 drop out before reaching the finish line.
And that's partly what attracts athletes like Kershaw. "I'm more drawn to it by the fact that there's so much carnage, that people don't finish, that it's so tough."
There wasn't as much carnage this year. The temperature at the start was a relatively balmy minus twelve degrees Celsius. And racers benefitted from the conditions. Minnesotan Casey Krueger smashed the ski record by 14 hours, finishing in just over 22 hours.
The first woman biker across the finish line - Eszter Horanyi from Colorado - took a bit over 18 hours to break the women's record by two hours, just two-and-a-half hours behind the winning biker.
And Jason Buffington, the doctor from Duluth, set a new course record for runners. He finished in just over 37 hours.