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)

Fans of Germany react as they watch the Euro 2016 match between France and Germany in France at a public screening of the match in Berlin, Germany, July 7, 2016. France won 2-0. (Photo by Hannibal Hanschke/Reuters)

“Hippo Chase”. As we approached the camp the Selinda Reserve of northern Botswana our boat passed by a hippo resting in the water. I turned around to look back at the boat's wake and saw the hippo charging after us! The hippo must've been tired from an all-nighter because he was angry. He rose way up out of the water three times trying to chase our boat! He was coming at us with such force that he created a wake of his own. Photo location: Selinda Reserve of northern Botswana. (Photo and caption by Curtis Simmons/National Geographic Photo Contest)

A woman poses for a photograph in front of a 3D painting at HK 3D Museum in Hong Kong June 26, 2014. (Photo by Tyrone Siu/Reuters)

A rose lies on a plastic sheet covering a victim of a Malaysian Airlines Boeing 777 plane which was downed on Thursday near the village of Rozsypne, in the Donetsk region July 18, 2014. International prosecutors investigating the downing of flight MH17 over eastern Ukraine in 2014 say the missile that hit the plane was fired from territory controlled by Russian-backed rebels. They said the missile launcher was brought into Ukraine from Russia. All 298 people on board the Boeing 777 died when it broke apart in midair flying from Amsterdam to Kuala Lumpur. The JIT investigation's findings are meant to prepare the ground for a criminal trial. (Photo by Maxim Zmeyev/Reuters)

The northern lights, or aurora borealis, appear in the sky over the Bamburgh lighthouse at Stag Rock in Northumberland, England on September 28, 2016. (Photo by Owen Humphreys/PA Wire via ZUMA Press)

This image made available by the Johns Hopkins University Applied Physics Laboratory on Wednesday, May 31, 2017 depicts NASA's Solar Probe Plus spacecraft approaching the sun. On Wednesday, NASA announced it will launch the probe in summer 2018 to explore the solar atmosphere. It will be subjected to brutal heat and radiation like no other man-made structure before. (Photo by Johns Hopkins University Applied Physics Laboratory via AP Photo)

Mercury and Maia, fueled and overhauled, are waiting in the Tay at Dundee, for favorable weather to start the flight to the Cape, a distance of 6,370 miles. The composite machine moored in the Tay River, at Dundee, on September 23, 1938. (Photo by AP Photo)