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)

A seller dressed as Snegurochka (The Snow Maiden) shows Christmas toys at the Christmas decorations set in the Moscow GUM State Department store in Moscow, Russia in Moscow, Russia, Monday, December 16, 2019. (Photo by Pavel Golovkin/AP Photo)

Horses eat hay and rest on straw at a stud farm in Wehrheim near Frankfurt, Germany, Tuesday, December 5, 2023. (Photo by Michael Probst/AP Photo)

A model stands in a light backstage after presenting the collection by Russian designer Alena Akhmadullina at The Russian State Library, one of the three national libraries of Russia, the largest library in the country, largest in Europe, in Moscow, Russia, on Wednesday, November 29, 2023. (Photo by Alexander Zemlianichenko/AP Photo)

Performers take part in a procession as part of the Samhuinn Fire Festival on October 31, 2024 in Edinburgh, Scotland. The Samhuinn Fire Festival is a modern take on an ancient Celtic festival, marking the transition between summer and winter with fire-dancing, drums, acrobatics, and theatre performances. (Photo by Jeff J. Mitchell/Getty Images)

A woman in traditional Malaysian attire welcomes leaders to the official dinner at the venue of the Association of Southeast Asian Nations (ASEAN) Summit in Kuala Lumpur, Malaysia November 21, 2015. (Photo by Jorge Silva/Reuters)

A restaurant worker wearing a mask cleans the windows as the restaurant reopens amid the COVID-19 pandemic in Sao Paulo, Brazil, Monday, July 6, 2020. Bars, restaurants and beauty salons were allowed to re-open Monday after over three months of quarantine, and are required to observe preventative measures and reduced operating hours only during the day. (Photo by Andre Penner/AP Photo)

Members of the U.S. Army's Golden Knights parachute team perform during the Bethpage Air Show as part of a Memorial Day event at Jones Beach on Long Island, New York, U.S., May 25, 2019. (Photo by Johnny Milano/Reuters)