We take a look at vertical running, a unique urban sport where athletes race up the stairs of tall buildings and landmark towers. Although it is an easy sport to take up, it pushes runners to extremes. The first ever vertical running race reportedly took place at the Eiffel Tower in 1905. One of the most famous races, first held in 1978, involves climbing the stairs of the Empire State Building. Around the world, more than 100,000 people take part in the sport. Recently, it is growing in popularity in Japan, and one Japanese runner is now among the world's best.