People cool off in a fountain outside the Southbank Centre on June 30, 2015 in London, England. The UK is currently experiencing a heatwave, with temperatures of 35 degree celsius forecast tomorrow in some parts of the country. The extreme heat has already seen train cancellations and a health warning has been issued. (Photo by Rob Stothard/Getty Images)

Russian soldiers write home while serving on the Eastern Front during World War I, circa 1915. (Photo by Hulton Archive/Getty Images)

Sculptures entitled “The Rising Tide” by British sculptor Jason deCaires Taylor are seen beside the River Thames in front of the Houses of Parliament and the London Eye ferris wheel in London, September 3, 2015. The representations of four horses and riders are fully visible at low tide but become immersed underwater twice a day as the Thames rises to reach full tide. The installation will be on display throughout September as part of the annual Totally Thames festival. (Photo by Toby Melville/Reuters)

A towel with a print of the Nigerian naira is displayed for sale at a street market in the central business district in Nigeria's commercial capital Lagos February 4, 2016. (Photo by Akintunde Akinleye/Reuters)

Policewomen take a picture inside a tunnel under the Chao Phraya river at a Mass Rapid Transit subway station in Bangkok, Thailand, December 14, 2015. (Photo by Athit Perawongmetha/Reuters)

Turkana tribesmen walk with guns to protect their cattle from rival tribes near Baragoy, Kenya August 4, 2017. (Photo by Goran Tomasevic/Reuters)

Spectators pass through security screening ahead of the New Year's Eve celebration in Times Square in New York, on Sunday, December 31, 2017. (Photo by Peter Morgan/AP Photo)

An armed forces members patrol during an operation against drug dealers in Vila Kennedy slum in Rio de Janeiro, March 7, 2018. (Photo by Pilar Olivares/Reuters)