A guard stands at Dityatki checkpoint, marking a 30 km (18-mile) zone around the Chernobyl nuclear power plant April 23, 2013. Ukraine will mark the 27th anniversary of the Chernobyl disaster, the world's worst civil nuclear accident, on April 26. (Photo by Gleb Garanich/Reuters)

In this photo taken Tuesday, February 2, 2016, asylum seekers gather round a fire as they cook a meal overlooking the temporary Altnes camp refugee camp on the island of Seiland, northern Norway. Waiting for their asylum claims to be processed, hundreds of people in emergency shelters in Hammerfest and neighboring towns are slowly getting used to the extreme climate and unfamiliar customs of the High North. They say they have adapted to the cold – the temperature rarely drops below minus 10 degrees C (14 F) along the coast, though it gets much colder further inland. It’s the darkness that throws them off. (Photo by Alastair Grant/AP Photo)

Lawmakers from the main opposition Republican People's Party (CHP) and ruling AK Party (R) scuffle during a debate on a legislation to boost police powers, at the Turkish Parliament in Ankara, Turkey, February 19, 2015. (Photo by Reuters/Stringer)

Women carry gas cylinders to fill them at a distribution point in Cairo January 19, 2015. Egypt is going through its worst energy crisis in decades and is seeking fresh sources of natural gas, which powers most of its homes and factories. (Photo by Mohamed Abd El Ghany/Reuters)

A couple takes a selfie as Hoa Binh hydroelectric power plant opens the flood gates after a heavy rainfall caused by Talas typhoon in Hoa Binh province, outside Hanoi, Vietnam July 20, 2017. (Photo by Reuters/Kham)

National Geographic photographer Steve Winter has spent most of his adult life shooting wild cats. Photo: A 14-month-old cub, cooling off in a pond, is riveted by a deer that appeared near the shore. Tigers are powerful swimmers; they can easily cross rivers four to five miles wide and have been known to swim distances of up to 18 miles. (Photo by Steve Winter/National Geographic)

Wedges of an orange generate enough current and electrical juice – 3.5 volts – to power an LED. The fruit’s citric acid helps electrons flow from galvanized nails to copper wire in this 14-hour exposure. This image was published in September’s Visions of Earth, a trio of photos that appear in each issue of National Geographic. (Photo by Caleb Charland/National Geographic)

A 10 km prologue dog sled race on the eve of the 2018 Beringia dog sled race in Russia' s Far East; the prologue race is part of the 2018 Russian Winter Dog Powered Sports Cup in Petropavlovsk-Kamchatsky, Russia on March 4, 2018. (Photo by Yuri Smityuk/TASS)