The summit of one of Europe's tallest mountains glows like the flame of a candle as the sun rises on a clear morning. The Matterhorn, famous for appearing on bars of Toblerone chocolate bars, reflects the vivid orange light at 5.37am. (Photo by Margarethe Jaeger/Solent News & Photo Agency)

A recruitment drive during the First World War at Trafalgar Square, London. (Photo by Central Press/Getty Images)

National Guardsmen are put through riot training in Boston's Commonwealth Armory on Friday, October 18, 1974. Massachusetts Gov. Francis W. Sargent called up the guard to quell school violence, but the city has been relatively calm and the guard has remained in the armories. (Photo by JWG/AP Photo)

Two robotic legged squad support system (LS3) machines by the Defense Advanced Research Project Agency follow a technician during a demonstration at Joint Base Myer-Henderson Hall, Virginia, on September 10, 2012. (U.S. Marine Corps photo by Sgt. Mallory S. VanderSchans via The Atlantic)

Following a large air raid by German bombers, local people, coming out of their air raid shelters found a bus standing on end in a large crater, in Balham, London, October 15, 1940. (Photo by AP Photo)

Maj. Matt O'Donnell of Glenelg, MD shields himself from rotor wash as Ospreys carrying the delegation of U.S. Secretary of Defense Leon Panetta leaves on March 14, 2012 at Forward Operating Base Shukvani, Afghanistan. Panetta is also scheduled to meet with President Karzai during his two-day visit to Afghanistan. The trip comes as the Taliban has vowed revenge following this past weekend's killing spree by a U.S. soldier who is accused of killing 16 civilians in Afghanistan. Most of those killed were children and women. (Photo by Scott Olson/Getty Images)

People who fled the violence from Islamic State-controlled northern Syrian town of al-Bab arrive in the rebel-held outskirts of the town, Syria February 3, 2017. (Photo by Khalil Ashawi/Reuters)

Participants lay down on a street covered by coloured powder during the annual Bangkok Color Run in Bangkok, Thailand April 1, 2017. (Photo by Athit Perawongmetha/Reuters)