The Northern Lights glow green in a spectacular light show above a field of trees frozen solid with snow. (Photo by Jaak Sarv/Solent News & Photo Agency)

Performers line up during the closing ceremony of the 2014 Winter Olympics, Sunday, February 23, 2014, in Sochi, Russia. (Photo by Darron Cummings/AP Photo)

Breitling Wingwalkers aircrafts perform during an aerobatic display at an air show in Zhengzhou, Henan province, China, September 25, 201. (Photo by Reuters/Stringer)

A boy looks for sellable materials close to the burning garbage in the permanent waste land under Sylhet City Corporation for a living, on January 15, 2015. (Photo by Md. Akhlas Uddin/Pacific Press)

In this Sunday, March 29, 2015 photo, a young performer from the Parinacochas district of Ayacuhco dances in the Vencedores de Ayacucho dance festival, in the Acho bullring in Lima, Peru. The performers sang in the Quechua language, portraying the planting of corn and potatoes. From very young children to elderly people dressed as farmers, tigers, and foxes, as well as members of auto-defense groups, soldiers and police officers, to portray the violence that tore apart their families and communities. (Photo by Rodrigo Abd/AP Photo)

A military police officer aims his gun to a demonstrator during a protest in Brasilia, Brazil, Tuesday, April 7, 2015. The officer had picked up a bottle that was thrown towards them but did not fire his gun and backed away. Thousands of workers have staged rallies in 12 cities across Brazil to protest against a proposed law that would allow companies to outsource their labor force. (Photo by Eraldo Peres/AP Photo)

Fish are thrown onto a truck during a winter fishing festival held on the frozen Wulungu Lake in Fuhai county, Xinjiang Uighur Autonomous Region, January 18, 2015. (Photo by Reuters/China Daily)