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)

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 tourist struggles in the water, before standing up and wading away, after a wave strengthened by the influence of Typhoon Dujuan hit a river bank in Hangzhou, Zhejiang province September 29, 2015. (Photo by Reuters/China Daily)

People stand in the middle of the graves and in front of giant kites in the cemetery of Santiago Sacatepequez, Guatemala, November 1, 2015. Dating back 116 years, the tradition of flying kites in the cemetery of of Santiago Sacatepequez, integrates the Catholic feast of All Saints with ancient Mayan practices of honouring the dead. It is believed that the kites connect the living and the dead during the all saints day celebration. (Photo by Jorge Dan Lopez/Reuters)

Tibetan pilgrims walk outside the Sera Monastery in Lhasa, Tibet Autonomous Region, China, 11 September 2016. Sera Monastery is known as one of the “great three” Gelug university monasteries of Tibet founded in 1419. (Photo by How Hwee Young/EPA)

A girl plays on a pile of discarded flowers outside a market, the day after the Diwali celebrations in Mumbai, India October 31, 2016. (Photo by Shailesh Andrade/Reuters)

A resident gestures about the loss of his home following an overnight fire at the Addition Hills area in suburban Mandaluyong city east Manila, Philippines, Monday, November 14, 2016. Fire officials say the overnight fire destroyed hundreds of homes, rendered more than 5,000 people homeless and resulted in the death of two residents. (Photo by Bullit Marquez/AP Photo)

The People of Mah Meri tribe at home at Pulau Carey village on December 03, 2016 in Selangor, Malaysia. The Mah Meri people are one 18 tribes of Orang Asal (indigenous people) living in west Malaysia. The Mah Meri are know for thei wood carvings and also for the richness of their songs and dances. (Photo by Yuli Seperi/Sijori Images/Barcroft India)