anywhere from 75% to 99.99%, practically a killing blow to Nome. The only practical way to deliver more serum to Nome in the middle of the coldest winter in 20 years was by dog sled. A relay of respected mushers was organized to expedite the delivery, and Seppala (with lead dog Togo) was chosen for the most forbidding part of the trail. The serum was to be taken by train to Nenana, and from there relay teams would set out from Nome and Nenana, meeting in the middle at Nulato. The whole trail was 674 miles from Nenana to Nome, and Seppala was initially selected to cover the more than 400 miles from Nome to Nulato and back. Seppala's section of trail featured a dangerous shortcut across Norton Sound, which could save a full day of travel. It was decided that he was the most qualified of the relay mushers to attempt this shortcut. The ice on Norton Sound was in constant motion due to currents from the sea and the incessant wind. It ranged from rough hills of smashed-together ice to slippery "glare ice" polished by the wind, where it was difficult for the dogs to get a foothold. Small cracks in the ice could suddenly widen, and driver and team could be plunged into the freezing water. If the wind blew from the east, it could reach speeds as high as (windchill is the loss of body heat induced by temps of and at least winds), flipping over sleds, pushing dogs off course, and causing a windchill as low as . A sustained east wind could also push the ice out to sea, and a team caught on a drifting floe could find itself stranded on open water. Seppala had taken the shortcut across the Sound several times in his career; a less-experienced musher was likelier to lose not only his life and the lives of his dogs, but also the urgently needed serum. Seppala would cross the sound each way in the race to deliver the serum. Seppala set out from Nome on January 28—several days before he was due to meet the relay coming to Nulato. He crossed Norton Sound without incident. Meanwhile, the number of diphtheria cases in Nome continued to climb. To hasten delivery of the serum, more mushers were added to the relay. However, it was too late to inform Seppala that he