The wheels fell off Than Jung's sled earlier this week, so the Thai runner borrowed a hammer and wrench from a Whitehorse hotel to fix it before it was loaded into a trailer bound for Dawson City.

The road ahead could span 566 kilometres, so Jung's gear needs to be in top shape.

He is one of 19 people who on Friday started the 6633 Ultra, a foot race from Eagle Plains, Yukon, to Tuktoyaktuk, N.W.T. The route follows the Dempster Highway to Inuvik and the ice road thereafter.

Competitors have to be mostly self-sufficient: they pull all their supplies on sleds or carts with wheels, sleep in tents along the roadside and cook their own food, although there are optional bag drops in Fort McPherson and Inuvik.

Only 12 per cent of competitors finish

Martin Like, founder of the 6633 Ultra, says there is no shame in failing to complete the race as nature can sometimes overwhelm even the heartiest competitor. (Philippe Morin) only ten people have made it all the way to Tuktoyaktuk in the race's history. The fastest time was just over five days. Martin Like, from Wales, established the 6633 Ultra in 2007 and saysTuktoyaktuk

Like describes the 6633 Ultra as a "very, very tough race."

"Only 12 per cent of people who start this race actually finish it," he says. "I think this is one of the few races where there's no shame in not finishing. The conditions up there can be very extreme. The cold and wind are what takes out a lot of people."

Philosophers of the road

Many runners, including Jung, are doing the ultra to raise money.

"I want to challenge myself and I am running for charity," Jung said. "I am planning to build a library for an orphanage house in Thailand. I have something like 300 people subscribing for donations."

Daithí Ó Murchú's motivation is more philosophical. The Irelander said he's interested in the places the mind goes when fighting pain and exhaustion.

He said it was the right time for him to do this race.

"Universally, the energies are right. The vibrations are right. It is probably the most difficult race in the world and I felt very deeply within myself and felt this was the time to honour Mother Earth. "

Most competitors pack the equivalent of a backpack, but this year there's a new approach: Mike Denoma travelled from London, England, with a large red shell on bicycle wheels. It's a mix between a rickshaw and camping trailer and it's made of lightweight carbon fibre.

"If it's cold you can get right in it and sleep," he said. "It's good to 30 below."

Denoma said it remains to be seen how much the Arctic wind will push against the trailer.

"This is either a good idea or massive stupidity, we'll see which one," he added with a laugh.

Competitors come from around the world, but there are no Canadians in the running this year.

Medical team keeps watch

This is Katie Rollins' second time working as part of the medical team that keeps an eye on the health of the runners. She's a doctor from Nottingham, UK. (Philippe Morin) This is Dr. Katie Rollins' second time as part of the medical team that examines runners' feet, faces and fingers.

"The racers get pretty tired, it's obviously it's a very long way to go," said the doctor from Nottingham, UK.

"Often they don't notice they're starting to get numbness or cold fingers and and that's the big thing to watch out for. We like to keep an eye on them, make sure all the exposed skin is covered."

Rollins has the power to scratch racers from the ultra. She said that power isn't used often but the medical team has the final word.

"I think everyone's very driven, very motivated," she surmised about why people compete.

"It's a challenge, we see them struggle, they have to work very hard, and it's not something you come across in daily life. It gets you away from the comforts of daily living and I think it's a fantastic challenge."

The 6633 Ultra is named for the location of the Arctic Circle at 66°33′ North latitude.