The quadruple axel — a 4½ rotation jump — has never been completed in competitive figure skating.

It’s a demanding feat, requiring strength, poise, stamina and confidence. It is one of the very few remaining challenges for Japanese figure skater Yuzuru Hanyu, who retired from competitive skating this week after an extraordinary career. He is considered one of the greatest figure skaters ever, and his departure leaves a gaping hole in the sport. It will be filled eventually, but he will be missed.

Born in Sendai in 1994, Hanyu began skating at the age of 4, and he made his junior international debut nine years later at the age of 13. A victory in Japan’s Junior Championships in 2008 allowed him to compete the following year in the World Junior Championships. He made his senior international debut two years after that and won the bronze medal in his first world championship appearance in March 2012.