The ten-kilometer skate pursuit climb in Val di Fiemme, Italy, is one of the most grueling courses on cross-country skiing’s World Cup circuit. After a few fast, mostly flat kilometers, athletes push and glide 1,300 vertical feet to the finish line midway up Alpe Cermis, an alpine ski hill. At its steepest, the grade reaches 44 percent. “Spectators on their alpine touring gear will have their heel risers in the highest position,” says Simi Hamilton, a member of the men’s U.S. Nordic Ski Team. “That’s always a reminder of how insane it is.” The course is the last stage of the Tour de Ski, an annual series of eight World Cup races in ten days, one of the most difficult events in the sport.