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 September 16, 1983 file photo, actor and bodybuilder Arnold Schwarzenegger shows off his new U.S. citizenship papers as Maria Shriver, daughter of Sargent and Eunice Shriver, looks on at the Shrine Auditorium in Hollywood, Calif. (Photo by Wally Fong/AP Photo)

Employees work at a sorting centre of Zhongtong (ZTO) Express ahead of the Singles Day shopping festival, Chaoyang District, Beijing, November 8, 2015. On China's giant Singles Day internet shopping festival, the country's delivery firms are stretched so thin that they are looking for tie-ups, listings and new investors to husband their resources. (Photo by Jason Lee/Reuters)

A truck lays overturned onto a car following strong winds at a Kurnell industrial park after a rare tornado hit the Sydney suburb December 16, 2015. The tornado hit Sydney on Wednesday with destructive winds above 200 km an hour (125 mph) and cricket ball-sized hail, bringing down trees and power lines, tearing off roofs, overturning vehicles and causing flash flooding. (Photo by Jason Reed/Reuters)

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 string is used to ensure that the skirts of female members of the honour guard are in the same height from the ground as they prepare for a welcoming ceremony for Congo Republic President Denis Sassou Nguesso outside the Great Hall of the People in Beijing, China, July 5, 2016. (Photo by Jason Lee/Reuters)

Actors dressed as Santa Claus take a refreshing bath at Bellevue Beach north of Copenhagen, Denmark, on July 24, 2018, as they take part in the World Santa Congress, an annual two-day event held every summer in Copenhagen. (Photo by Mads Claus Rasmussen/AFP Photo/Ritzau Scanpix)

Actors Alexandra Shipp and Vanessa Hudgens attend a premiere screening for “Tick, Tick . Boom” during the opening night of AFI Fest at TCL Chinese theatre in Los Angeles, California, U.S. November 10, 2021. (Photo by Mario Anzuoni/Reuters)