Participants hang from ropes as they learn how to use safety gears at an experience centre for construction safety training in Beijing, September 5, 2014. The centre which opened in June 2014 was established to educate workers about construction safety. According to the centre, about 1,000 construction workers undergo 3-hour classes which comprises of 33 modules every month. (Photo by Kim Kyung-Hoon/Reuters)

Dhruva Aliman does a back somersault as he dismounts from a slackline on the beach at Santa Monica, California on October 19, 2014. Slackline is a balance sport where a line of webbing is adjusted at different levels of tension between two anchor points and participants (called slackers) perform tricks and stunts including somersaults. (Photo by Mark Ralston/AFP Photo)

Russia's Maria Stavitskaia performs during the ladies short program at the Rostelecom Cup ISU Grand Prix of Figure Skating in Moscow November 14, 2014. (Photo by Grigory Dukor/Reuters)

A monkey is led by its trainer Qi Defang during training for a circus in Suzhou, Anhui province, November 29, 2014. Suzhou is known as the hometown of circus troupes in China and has more than 300 circus troupes. (Photo by William Hong/Reuters)

Locals sort through garbage at a roadside dump in New Delhi on January 10, 2017 The national capital has been reeling under tonnes of garbage after sanitation workers went on a strike demanding payment of pending salaries. (Photo by Chandan Khanna/AFP Photo)

A participant prepares for the Red Bull Air Race World Championships in Abu Dhabi, United Arab Emirates on February 6, 2017. (Photo by Red Bull Content Pool/SIPA Press/Rex Features/Shutterstock)

Rougine, a 19-year-old female Arab fighter among the Syrian Democratic Forces, made up of US-backed Kurdish and Arab fighters, stands in fatigues embracing another colleague near the village of al-Torshan, 20 km on the outskirts of Raqa on February 6, 2017. (Photo by Delil Souleiman/AFP Photo)

The Breitling DC-3, HB-IRJ, plane fly escorted by Italian Air Force's aerobatic demonstration team, the Frecce Tricolori, during the last stage of its world tour two days ahead of the International Sion Air Show, in Sion, Switzerland, 13 September 2017. From March to September 2017, the Breitling DC-3 from 1940 has done a grand world tour in several stages, over 28 countries and flies 50000km, an amazing accomplishment for this legendary plane that will be celebrating its 77th birthday. (Photo by Christian Merz/EPA/EFE)