Gas-masked men of the British Machine Gun Corps with a Vickers machine gun during the first battle of the Somme, 1916. (Photo by General Photographic Agency/Getty Images)

A man sleeps on a wheelbarrow outside a closed shop at a market during the first day of a ceasefire in Yemen's capital Sanaa April 11, 2016. (Photo by Khaled Abdullah/Reuters)

Chocolate-lovers have gathered for the first Salon du Chocolat fashion show and festival in Brussels. The festival takes place in 19 cities worldwide, but this year marks the first Salon du Chocolat in Brussels. The main feature of the festival was a fashion show, where chefs and designers collaborated to create chocolate outfits. (Photo by Farrukh Younus/Implausibleblog)

An ancient oak tree sits in a Shropshire field as the first signs of Spring begin to show across the United Kingdom on February 19, 2014 in Bridgnorth, England. (Photo by Christopher Furlong/Getty Images)

A woman sits with a Great Dane during the first day of the Crufts dog show in Birmingham, central England March 6, 2014. (Photo by Darren Staples/Reuters)

The contest began in 2007, and since then photographers from more than 70 countries have entered. Photo: Michael O'Neal of San Francisco won first place in the animals category in the 2014 iPhone Photography Awards. (Photo by Michael O'Neal)

Soviet Army soldiers sit on their tanks in front of the Czechoslovak Radio station building in central Prague during the first day of Soviet-led invasion to then Czechoslovakia on August 21, 1968. (Photo by Libor Hajsky/Reuters)

The first four finalists are selected during the 66th Miss Universe pageant at Planet Hollywood hotel-casino in Las Vegas, Nevada on November 26, 2017. (Photo by Steve Marcus/Reuters)