A hot air-balloon flies during the International Lorraine Mondial Air Balloons Festival, in Chambley-Bussieres, eastern France, Friday, July 26, 2013. (Photo by Mathieu Cugnot/AP Photo)

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)

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)

Aleksandra Krunic, of Serbia, drops to the court after defeating Petra Kvitova, of the Czech Republic, during the third round of the 2014 U.S. Open tennis tournament, Saturday, August 30, 2014, in New York. (Photo by Kathy Willens/AP Photo)

Dancers perform during the Opening Ceremony of the Rio 2016 Olympic Games at Maracana Stadium on August 5, 2016 in Rio de Janeiro, Brazil. (Photo by Jamie Squire/Getty Images)

Bradley Hemmings and Jenny Sealey MBE pose for the camera with a Giant Iron Man on the set of Iron Man at the Greenwich and Docklands Festival in St Alfege Park on June 24, 2011 in London, England after they are announced by LOCOG as the Artistic Directors for the Paralympic Games Opening Ceremony. (Photo by Christopher Lee/Getty Images)

Jensen a two-year-old False Map turtle, from the Mississippi River, gets used to his new home at Blackpool Sea Life Centre on February 13, 2012 in Blackpool, England. (Photo by Christopher Furlong/Getty Images)

A man photographs the main engines of the space shuttle orbiter Discovery on display at the Udvar-Hazy Smithsonian National Air and Space Annex Museum in Chantilly, Virginia August 28, 2015. Shuttle Discovery had 27 years of service and flew 39 times before being retired in 2011. (Photo by Gary Cameron/Reuters)