Vietnam’s Son Doong cave, the largest in the world, could hold a 40-story skyscraper inside. The pristine ecosystem has its own river and jungle. Despite its size, Son Doong wasn’t discovered until 1991. It was lost again for nearly two decades and was fully explored for the first time in 2009. (Photo by Jason Speth/HuffPost)

A Bosnian special forces soldier returns fire 06 April 1992 downtown Sarajevo as he and civilians come under fire from Serbian snipers. The Serb extremists were shooting from the roof of a hotel at a peace demonstration of some of 30,000 people as fighting between Bosnian and Serb fighters escalated in the capital of Bosnia- Hercegovina. (Photo by Mike Persson/AFP Photo)

An anti-coup protester holds a Burkinabe flag in Ouagadougou, Burkina Faso, September 18, 2015. Security forces in the capital of Burkina Faso fired in the air on Friday to disperse demonstrators who burned tyres and blocked neighborhood streets to protest at a military coup this week that derailed a democratic transition. The head of a military junta in Burkina Faso which took power on Thursday has freed interim President Michel Kafando and two of his ministers, state television said on Friday. (Photo by Joe Penney/Reuters)

A bulldog dressed as Marilyn Monroe poses for a photograph during the annual Tompkins Square Halloween Dog Parade in the Manhattan borough of New York City, October 24, 2015. (Photo by Stephanie Keith/Reuters)

Hindu devotees laugh as they watch the religious festival of Lathmar Holi, where women beat the men with sticks, in the town of Barsana in the Uttar Pradesh region of India, March 17, 2016. During Lathmar Holi the women of Barsana beat the men from Nandgaon, the hometown of Krishna, with wooden sticks in response to their teasing. (Photo by Cathal McNaughton/Reuters)

In his series of untitled photographs Nobody Claps Anymore, the Mexican-American photographer Alec Dawson portrays ordinary people in their homes in a downbeat, ultra-stylised manner. Staring into space as they watch TV or water the garden, the real-life subjects of photographer hint at vast reserves of boredom and regret. (Photo by Alec Dawson)

In this Wednesday, November 15, 2012 file photo, a young girl in her colorful dress reaches out to greet a Pakistani policeman securing the road outside Kainat Riaz's home in Mingora, Swat Valley, Pakistan. (Photo by Anja Niedringhaus/AP Photo)

A Sikh devotee carries on over-sized religious sword during a religious procession to celebrate the birth anniversary of the first Sikh Guru or master, Sri Guru Nanak Dev Ji, the founder of Sikhism in Amritsar, India, 05 November 2014. The birth anniversary of Guru Nanak Dev Ji will be observed on 06 November. (Photo by Raminder Pal Singh/EPA)