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)

Two girls wearing traditional Sevillian dresses talk during the “Feria de Abril” (April Fair) in Sevilla on April 30, 2017. The fair dates back to 1847 when it was originally organized as a livestock fair but has turned into a week of flamenco dancing, music and bullfighting. (Photo by Cristina Quicler/AFP Photo)

Two life-size Victorian style dolls shocked Londoners morning December 3, 2015 as the creepy pair popped up at commuter hotspots across the capital to mark the launch of the world's first psychological theme park ride created by Derren Brown, coming to Thorpe Park Resort in 2016. (Photo by Matt Alexander/PA Wire)

Two people wearing animal costumes join others as people gather to celebrate the Winter Solstice at Stonehenge in England Tuesday December 22, 2015. The winter solstice is celebrated by many people around the world as the beginning of the return of the sun. (Photo by Ben Birchall/PA Wire via AP 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)

Idania Espanola, 63, poses for a photo showing a bra that she will adapt into two face masks, amid the spread of the new coronavirus in Cojimar, Cuba, east of Havana, Tuesday, March 31, 2020. (Photo by Ramon Espinosa/AP Photo)

Russian Tu-95MS strategic bombers fly in formation during rehearsals for the Victory Day military parade, with St. Basil's Cathedral seen in the foreground, in central Moscow May 5, 2015. Russia will celebrate the 70th anniversary of the victory over Nazi Germany in World War Two on May 9. (Photo by Tatyana Makeyeva/Reuters)

Emergency responders respond to the scene of a 565-foot-tall crane that toppled and flipped upside down, stretching along nearly two city blocks in downtown Manhattan in New York, February 5, 2016. The massive construction crane collapsed in lower Manhattan during a swirling snowstorm on Friday, killing one person and crushing a line of parked cars in the first accident of its kind in New York City since 2008. (Photo by Brendan McDermid/Reuters)