A bunker at aformer Swiss artillery fortress called Heldsberg stands near the town of St. Margareten, Switzerland March 22, 2015. Heldsberg fortress, located on the Swiss-Austrian border near the River Rhine and Lake Constance was built from 1938 to 1940 and remained in military use until 1992. Since 1993 it is open to the public as Fortress Museum Heldsberg. (Photo by Arnd Wiegmann/Reuters)

People stroll alongside one of the hills of the Tio Pio park in Madrid, Monday, January 25, 2016. The park is a high viewpoint frequented mostly by locals due to its view of the Spanish capital skyline. (Photo by Francisco Seco/AP Photo)

South Korea's Black Eagles aerobatics team performs an aerial display forming a Ying and Yang logo during the Singapore Airshow at Changi exhibition center in Singapore on February 16, 2016. (Photo by Roslan Rahman/AFP Photo)

A photograph of Cuba's former President Fidel Castro decorates a wall inside a state-run market in Havana, March 8, 2016. (Photo by Alexandre Meneghini/Reuters)

Refugee children play with a stuffed toy at a muddy makeshift camp at the Greek-Macedonian border, near the village of Idomeni, Greece March 15, 2016. (Photo by Alexandros Avramidis/Reuters)

A woman sports a rainbow painted hand at the annual PrideFest celebration at Civic Center Park in Denver, Colorado June 18, 2016. (Photo by Andy Cross/The Denver Post)

Praia du Forte, Bahia. “This was the lead picture for a National Geographic story on Bahia, because it was both mysterious as well as a “geography” picture showing where the slaves arrived from Nigeria. Beach pictures seem like they should be easy to take, but for me they are actually quite difficult. Too easy to fall into cliche. When I saw the kid coming on the horse, I quickly ran to see if I could make something with a woman in a bikini that was not a bikini shot”. (Photo by David Alan Harvey/The Guardian)

A woman wearing a mask walk in a street in a Yonghegong Lama temple compound seen shrouded in smog in Beijing, China, 30 November 2015. Beijing issued an orange alert for heavy smog 30 November, the highest level this year. Authorities in the Chinese capital warned of 'severe pollution' and advised the capital's 20 million inhabitants to stay indoors. (Photo by How Hwee Young/EPA)