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)

A soldier helps another with her ear protection at the Terningmoen Camp in Elverum, Norway on March 23, 2017. Soldiers demonstrate their skills and tactics during a contract drill as they train to become part of the world's first all-female special forces unit, the Jegertroppen or “hunter troops”. (Photo by Carolina Reid/NBC News)

From stuffed elephants to picnics under power lines, Leipzig-born photographer Frank Herfort coaxes magical, colour-saturated tableaux from intriguing slices of everyday Russian life. Here: A lonely dog watches the mailboxes in a typical Moscow residential block. (Photo by by Frank Herfort/The Guardian)

People use snowshoes during a tour of the Great Kemeri Bog, Latvia, October 17, 2015. The Kemeri bog is more than 8,000 years old and is one of the largest dry moss swamps in the Baltics. A more than decade-long joint restoration of the bog by the European Union and the Latvian government helped the bog recover its high moss marshes, damp black alder forests, floodplain meadows and seaside lakes – features now considered rare in Europe due to industrialisation. (Photo by Ints Kalnins/Reuters)

Flames from the Erskine Fire engulf a trailer near Weldon, California, U.S. June 24, 2016. (Photo by Noah Berger/Reuters)

A man prepares tea at a roadside shop on a flooded street in Kolkata, India, August 11, 2016. (Photo by Rupak De Chowdhuri/Reuters)

Reverend Simon Davis prays with the dancers before the Abbots Bromley Horn Dance in Abbots Bromley, Britain, September 12, 2016. The Abbots Bromley Horn Dance is an English folk dance dating back to the Middle Ages. (Photo by Darren Staples/Reuters)

A woman dressed as a character from the computer game “World of Warcraft” stands in a forest near the village of Sosnova, Czech Republic, April 30, 2016. (Photo by David W. Cerny/Reuters)