A family take a souvenir picture next to giant male panda Xiao Liwu, who was born at the San Diego Zoo and will be repatriated to China with his mother Bai Yun, bringing an end to a 23-year-long panda research program in San Diego, California, U.S., April 18, 2019. (Photo by Mike Blake/Reuters)

A Darjeeling Himalayan Railway steam train, which runs on a 2 foot gauge railway and is a UNESCO World Heritage Site, passes by a market in Ghum, India, June 27, 2019. (Photo by Ranita Roy/Reuters)

Poland's Piotr Lisek celebrates after winning in the Men's pole vault during the IAAF Diamond League competition on July 12, 2019 in Monaco. (Photo by Eric Gaillard/Reuters)

Men ride a scooter through a flooded street after heavy rains in New Delhi, India, August 6, 2019. (Photo by Adnan Abidi/Reuters)

German troops in Stalingrad, USSR. September 10, 1942. (Photo by TASS)

Ammunition airlift into besieged Khe Sanh, Operation Pegasus, April 1968. (Photo by Larry Burrows/AP Photo)

German Nazi leader Adolf Hitler (1889 - 1945) enjoys a quiet picnic between meetings, circa 1933. Picture 196 of a series of collectable images published in Germany during the Nazi period, entitled “Deutschland Erwacht” (Germany Awakes). (Photo by Heinrich Hoffmann/Hulton Archive/Getty Images)

An air raid drill at Seattle College caught students William Bates, left, and Edna Mae Farrell, right, while they were eating their lunch in the school café, but they simply took shelter under the table and continued eating in Seattle, Washington, March 17, 1942. Air raid wardens said 350 students left their classrooms and assembled in shelters within one minute in first at the college. (Photo by AP Photo)