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)

In this April 4, 2017 photo, Zoologist Martha Llanes caresses baby chimpanzee Anuma II, left, while Ada hangs on to her leg, at Llanes' apartment in Havana, Cuba. She has forgiven them every transgression. It's hard to stay angry at a baby chimpanzee when it clambers up your leg and into your arms and plants a kiss on your cheek in a plea for forgiveness. (Photo by Ramon Espinosa/AP Photo)

Models pose in designs from May Quant's collection on a street in London, England, on October 16, 1969. Grania, left, wears the "Shimmy Shimmy," a white rayon dress over matching pants with a shawl. Baba, center, wears "Razzamatazz," a jumpsuit featuring plastic sequins in blue, silver and red on nylon. Linda wears "Muffit," a pink minidress with an old English style floppy mobcap. (Photo by AP Photo)

A displaced Pakistani girl collects recyclable goods from a garbage to earn living for her family in Islamabad, Pakistan, Tuesday, January 19, 2016. (Photo by B.K. Bangash/AP Photo)

Food is placed on the tomb of pet dog Xixi at Baifu pet cemetery ahead of the Qingming Festival on the outskirts of Beijing, China March 26, 2016. The Chinese characters on the gravestone read: “Mommy's only good son”. (Photo by Jason Lee/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)

Norway's Karsten Warholm celebrates winning the men's 400m hurdles at the Diamond League in London, July 21, 2018. (Photo by Andrew Boyers/Action Images via Reuters)

A camel is pictured in front of the Saturn tracking complex at the Russian-leased Baikonur cosmodrome in Kazakhstan on October 9, 2018. An astronaut and cosmonaut are due to travel to the ISS on October 11 aboard a Russian Soyuz MS-10 spacecraft from the Baikonur Cosmodrome in Kazakhstan. (Photo by Kirill Kudryavtsev/AFP Photo)