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 Iranian girl walks at the Tochal ski resort, in north of Tehran, Iran on December 12, 2019. (Photo by Nazanin Tabatabaee/WANA via Reuters)

A woman jumps with balloons while she poses for a picture at a park in Kiev, Ukraine May 6, 2018. (Photo by Valentyn Ogirenko/Reuters)

A woman takes photos as Mount Agung volcano sends up another plume of smoke, seen from the Kubu subdistrict in Karangasem Regency on Indonesia's resort island of Bali on July 5, 2018. Mount Agung roared to life again on July 2, belching a plume of ash 2,000 metres (6,500 feet) high, as well as temporarily shuttering the airport and grounding hundreds of flights after erupting the week before. (Photo by Sonny Tumbelaka/AFP Photo)

A USGS geologist making observations of the fissure 8 lava channel at sunset is pictured in this July 3, 2018 fisheye lens handout photograph near the Kilauea volcano eruption in Hawaii, U.S. (Photo by USGS/Handout via Reuters)

A six-day-old hippopotamus is pictured next to his mother, Kara, aged 21, on September 12, 2012 at “Planet sauvage” (“Wild planet”) Zoo in Port-Saint-Pere, western France. The birth, a rare event for this species in captivity, occured on September 7, 2012 in the Zoo. (Photo by Frank Perry/AFP Photo)

Ten day old male Gorilla “Fataki” clings to his mother “Frala” during his media debut at Taronga Zoo June 4, 2003 in Sydney, Australia. Fataki was born on May 24, 2003 and was the fourth Gorilla to be born at Taronga Zoo since the opening of the Gorilla Forest in 1997. (Photo by Chris McGrath/Getty Images)

Durga Kami, 68, who is studying tenth grade at Shree Kala Bhairab Higher Secondary School, walks with his classmate Sagar Thapa, 14, as they head to school in Syangja, Nepal, June 5, 2016. (Photo by Navesh Chitrakar/Reuters)