Željava Air Base, situated on the border between Croatia and Bosnia and Herzegovina under Plješevica Mountain, near the city of Bihać, Bosnia, was the largest underground airport and military air base in the former Yugoslavia, and one of the largest in Europe. (Photo by Thomas Windisch/Exclusivepix Media)

Crew members work to secure a hot-air balloon after landing at the city of Luxor, south of Cairo, Egypt December 13, 2016. (Photo by Amr Abdallah Dalsh/Reuters)

National Geographic has created “Air, Land & Sea: the 50 greatest wildlife photographs” exhibition. Here: CT4 Crocodile cave on the Salamat river. Set up with Nathan Williamson last chip rain came while we were with the nomads. (Photo by Michael Nichols/National Geographic)

A handout picture provided by the press service of MORTON Group of Companies, show the balloon of Russian adventurer Fedor Konyukhov in the sky during his solo round-the-world balloon flight just after taking off from a spot near Northam, 96 kilometres north-east of Perth in Western Australia, 12 July 2016. Fedor Konyukhov took off from Northam on 12 July 2016 in attempt to beat the record of 13 days of American aviator Steve Fossett. (Photo by EPA/Morton Press Service)

Employees work at a sorting centre of Zhongtong (ZTO) Express ahead of the Singles Day shopping festival, Chaoyang District, Beijing, November 8, 2015. On China's giant Singles Day internet shopping festival, the country's delivery firms are stretched so thin that they are looking for tie-ups, listings and new investors to husband their resources. (Photo by Jason Lee/Reuters)

Tourists standing near the sea shore are hit by a wave, which surged past a barrier under the influence of Typhoon Chan-hom, in Qingdao, Shandong province, China, July 12, 2015. One of the most powerful typhoons to strike eastern China in decades disrupted air, rail and sea transport on Saturday after forcing the evacuation of more than a million people from the provinces of Zhejiang and Jiangsu, state media reported. (Photo by Reuters/Stringer)

Local residents crowd a swimming pool during a hot weather in Suining, Sichuan province, 27 July 2008. (Photo by Reuters/Stringer)

A rescuer saves a resident from a flooded area in Wuhan, Hubei Province, China, July 2, 2016. (Photo by Reuters/China Daily)