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)

Boys ride a motorbike on their way back home after taking a bath in a canal at Chachura village, in Uttar Pradesh April 4, 2012. (Photo by Parivartan Sharma/Reuters)

A young Afghan vendor waits for customers to buy sheep's heads at a roadside stall at the busy market in Kabul on November 29, 2015. (Photo by Wakil Kohsar/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)

Father Christmas reading with three young children. (Photo by Spencer Arnold/Getty Images). Circa 1915

Daniel Filip, Tech Lead Manager for Google Maps, carries the Trekker, a 15-camera device, while mapping the Inca citadel of Machu Picchu for Google Street View in Cuzco, Peru, August 11, 2015. (Photo by Pilar Olivares/Reuters)

Students from Cambridge University in England make their way home along Trinity Lane after celebrating the end of the academic year at a May Ball in Trinity College on June 18, 2019. (Photo by Joe Giddens/PA Images via Getty Images)

Students wearing mask hold hands to surround the St. Stephen's Girls' College in Hong Kong, Monday, September 9, 2019. Thousands of demonstrators in Hong Kong urged President Donald Trump to “liberate” the semiautonomous Chinese territory during a peaceful march to the U.S. Consulate on Sunday, but violence broke out later in the business and retail district as police fired tear gas after protesters vandalized subway stations, set fires and blocked traffic. (Photo by Kin Cheung/AP Photo)