Ultra-endurance race in the Yukon sub-Arctic (Arctic temperatures).The race is 430 miles (other categories include marathon, 100-miler and 300-miler). Race can be completed on foot, on cross-country skis, or on an adapted mountain bike (one discipline throughout).
I am going back in February this year. I'm one of only two people to have completed the 430-mile race on foot twice (2009 and 2011) and will be returning in 2013 to be the only person to have completed it three times (the 430-mile races takes place every other year, although the other distances are raced every year).
This is an excellent opportunity for those wanting to test their cold-weather skills in a pristine wilderness, with support available in an emergency. A book on the race can be found on Amazon (The Yukon Arctic Ultra, by Mark Hines).