US marines watch the mushroom cloud from an atomic explosion rise above the Yucca flats, Nevada during a US nuclear weapons test. (Photo by Keystone/Getty Images). USA, 1945

Some of London's telephone operators and engineers, trying out their gas masks at the anti-gas civilian training centre in Gloucester, where they were taught how to deal with gas attacks. (Photo by Harry Todd/Fox Photos/Getty Images). 13th February 1937

German field telegraph operators at work in Warsaw. (Photo by Hulton Archive/Getty Images). 1st October 1914

Zurab Gurielidze, a director of the Tbilisi Zoo, speaks to the media while standing next to a hippopotamus named Begi at the zoo in Tbilisi, Georgia, September 13, 2015. Tbilisi zoo, which was partly destroyed and some 300 animals escaped or died in the floods that also killed at least 19 people in June, was reopened on Sunday. (Photo by David Mdzinarishvili/Reuters)

A dressed reveler looks up to the famous cathedral in Cologne, Germany, Wednesday, November 11, 2015. Thousands of carnival supporters celebrate the traditional beginning of the carnival season in Germany. (Photo by Martin Meissner/AP Photo)

Two men try to load their just purchased Samsung 55" TV into a car, at a Best Buy store in Fairfax, Virginia on November 26, 2015, on a Black Friday sale that started a day earlier during Thanksgiving evening. The US holiday shopping season kicks off with "Black Friday" – the day after the Thanksgiving holiday – with a frenzy expected at stores around the country as retailers slash prices. (Photo by Paul J. Richards/AFP Photo)

Jassen Todorov, 40, from San Francisco is a professor of music at San Francisco State University, but he took to helicoptering in his spare time. While he was content to keep the panoramic views he witnessed to himself, Todorov has now paired his passion for flying with photography to capture some of the United States’ undiscovered vistas. Here: Grand Prismatic Spring, Yellowstone, Wyoming. (Photo by Jassen Todorov/Caters News)

People watch police destroy confiscated illegal firecrackers in Shenyang, Liaoning province, China, January 29, 2016. (Photo by Reuters/Stringer)