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)

Dancers wait at backstage as they prepare for a dance performance during the Bali Arts Festival in Bali, Indonesia, Wednesday, June 21, 2017. The resort island of Bali is holding a month-long annual Bali Arts Festival from June 10 to July 8. (Photo by Firdia Lisnawati/AP Photo)

A wildlife caregiver holds an orphaned wombat at the Native Wildlife Rescue center on January 29, 2020 in Robertson, Australia. The center has taken in many burned kangaroos and wallabies injured in recent bushfires. Wombat orphans are often rescued from the pouch of their mothers struck by vehicles. (Photo by John Moore/Getty Images)

Protesters wear gas masks in the front of the Romanian Environment Ministry in Bucharest March 4, 2019, to demand better air. Dozens of people holding placards reading “Our children don't breath air anymore” gather in the front of the Environment Ministry to ask for better breathable air in Bucharest. (Photo by Daniel Mihailescu/AFP Photo)

A robot, developed by start-up firm Asimov Robotics, holds a tray with face masks and sanitizer after the two robots were launched to spread awareness about the coronavirus, in Kochi, India, March 17, 2020. (Photo by Sivaram V/Reuters)

A demonstrator holds up her fist, in front of police officers during a protest, organised by Black Lives Matter Belgium, against racial inequality in the aftermath of the death in Minneapolis police custody of George Floyd, in central Brussels, Belgium on June 7, 2020. (Photo by Yves Herman/Reuters)

A Chinese ethnic Lisu honey hunter holds a large piece of wax from a hive while gathering wild cliff honey in a gorge on May 10, 2019 near Mangshi, in Dehong prefecture, Yunnan province China. (Photo by Kevin Frayer/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)