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)

A member of a local swimming club holds a Russian national flag as he dives in at the start of a 24-hour swimming marathon near the western Siberian city of Barnaul March 29, 2014. Thirty members of the club will swim in turn during the event, held to mark Crimea becoming part of Russia, local media reported. (Photo by Andrei Kasprishin/Reuters)

A passenger aircraft passes over a residential house as it prepares to land at London Heathrow Airport in west London on October 17, 2016. Britain's government is considering whether to approve a third runway at Heathrow or expand air capacity in southeast England at another airport such as London Gatwick. (Photo by Daniel Leal-Olivas/AFP Photo)

A sun bear reacts to triple-digit temperatures at the Henry Doorly Zoo in Omaha, Nebraska, where the temperature reached 103 degrees Fahrenheit on July 6, 2012. (Photo by Nati Harnik/Associated Press)

An Israeli army officer gives explanations to journalists during an army organised tour in a tunnel said to be used by Palestinian militants for cross-border attacks, July 25, 2014. (Photo by Jack Guez/Reuters)

Kumbuka, a male silverback gorilla sits next to the keeper's chalk board in his enclosure at London Zoo, Thursday, January 2, 2014. Home to more than 850 different species, zoo keepers welcomed in the New Year armed with clipboards as they made a note of every single animal. The compulsory annual count is required as part of ZSL London Zoo's zoo license, and every creature, from the tiny leaf cutter ants to the huge silverback gorilla is duly noted and accounted for. (Photo by Kirsty Wigglesworth/AP Photo)

Cyclists compete during the UCI BMX World Championships Elite Women's Moto Race on May 29, 2016 in Medellin, Antioquia department, Colombia. (Photo by Raul Arboleda/AFP Photo)

A woman stands in a gift shop in central Rason city, part of the special economic zone northeast of Pyongyang, in this August 30, 2011 file photo. North Korea is a militarized, male-dominated society, but it is women who are making the money as the insular nation allows an unofficial market-based economy to take shape. (Photo by Carlos Barria/Reuters)