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)

An Airbus A400M military aircraft lands at the ILA Berlin Air Show in Schoenefeld, south of Berlin, Germany, June 1, 2016. The Aerospace Exhibition at Schoenefeld Airport near Berlin takes place from June 1 till 4. (Photo by Fabrizio Bensch/Reuters)

Eight-year-old handicapped Chinese girl Qian Hongyan crawls with two home-made props and part of a basketball at Zhuangxia Village on January 5, 2005 in Luliang County of Qujing City, Yunnan Province, China. The girl who lost both of her legs in a traffic accident in 2000 at the age of three, struggles to live her life with a basketball as an underprop, “walking” between school and home by herself. The girl's story is widely reported in the country, and experts from China Rehabilitation Research Center has come to Qian's home to help her for rehabilitative treatment early 2005. (Photo by China Photos/Getty Images)

Students wearing mask hold hands to surround the St. Stephen's Girls' College in Hong Kong, Monday, September 9, 2019. Thousands of demonstrators in Hong Kong urged President Donald Trump to “liberate” the semiautonomous Chinese territory during a peaceful march to the U.S. Consulate on Sunday, but violence broke out later in the business and retail district as police fired tear gas after protesters vandalized subway stations, set fires and blocked traffic. (Photo by Kin Cheung/AP Photo)

Two baby orangutans play with each other at the wildlife department in Kuala Lumpur, Malayasia, October 19, 2015. The Malaysian wildlife department in July seized two baby Sumatran orangutans, found in duffel bags, from traffickers who were attempting to sell them to buyers in Malaysia. According to local media, the orangutans will be returned to Medan, Indonesia on Tuesday. The illegal wildlife trade is estimated to be $8 billion a year worldwide, according to TRAFFIC, a wildlife trade monitoring network. (Photo by Olivia Harris/Reuters)

A female participant armed with a bottle of ketchup attacks in the annual Vegetable Battle in Kreuzberg district near Oberbaumbruecke Bridge on August 28, 2011 in Berlin, Germany. The event pits Kreuzberg district residents againts Friedrichshain district residents for control of the Oberbaumbruecke, and the two sides pelt each other with rotten vegetables, other foods, ketchup, water guns and styrofoam bats until one side has pushed the other from the bridge. The event had originally been cancelled for today, but so many participants showed up anyway and began battling on the Kreuzberg side, where the situation escalated and began to threaten traffic, that police relented and let them later battle it out on the bridge. Kreuzberg claimed victory. (Photo by Sean Gallup/Getty Images)

A worker paints the Amazon Tall Tower Observatory (ATTO) in Sao Sebastiao do Uatuma in the middle of the Amazon forest in Amazonas state January 10, 2015. The Amazon Tall Tower Observatory is a project of Brazil's National Institute of Amazonian Research and Germany's Max Planck Institute and will be equipped with high-tech instruments and an observatory to monitor relationships between the jungle and the atmosphere from next July. According to the institutes, ATTO will gather data on heat, water, carbon gas, winds, cloud formation and weather patterns. (Photo by Bruno Kelly/Reuters)

A cat with a “stegosaurus spine” design cut into its fur is seen next to a dog at a pet shop, in Tainan, Taiwan June 19, 2016. Lee Mei-chen, Taiwanese hairdresser turned pet groomer, shaves paw prints, lion faces and characters from Japanese brand Hello Kitty on to animal backs. The teddy bear design is one of the most complex in the furry repertoire. Photo by Tyrone Siu/Reuters)