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)

This undated photo shows the Arctic National Wildlife Refuge in Alaska. (Photo by U.S. Fish and Wildlife Service/Getty Images)

A building emerges from the early morning fog in Belvedere farm in San Quirico d'Orcia, Tuscany, Italy. The beautiful pictures were taken by Alberto Di Donato who has a huge passion for the Tuscan landscape and is inspired by the Renaissance artists who painted the same landscapes. (Photo by Alberto Di Donato/HotSpotMedia)

Tourists walk amongst beach foam in the wake of cyclonic conditions at Currumbin Beach on December 15, 2020, after wild weather lashed Australia's Northern New South Wales and South East Queensland with heavy rain, strong winds and king tides. (Photo by Patrick Hamilton/AFP Photo)

A view shows a Lun-class Soviet-made ekranoplan, which is a naval craft and a ground-effect vehicle, on the Caspian Sea coastline in Derbent in the Republic of Dagestan, Russia on November 3, 2020. (Photo by Kazbek Basayev/Reuters)

A man walks on the road between Nouahibou and Nouakchott, where three Spanish aid workers were abducted in Mauritania, December 3, 2009. (Photo by Rafael Marchante/Reuters)

Rick Sorenson, owner of the Rio Ramaza Marina, wades in the Sacramento River after securing an old paddlewheel boat on his property as the river makes its way up the levee on Garden Highway in Sacramento, Calif., Monday, January 9, 2017. (Photo by Hector Amezcua/The Sacramento Bee via AP Photo)

Snow falls as a woman walks her dog in the village of Podolye, 70 kilometers (43 miles) east of St. Petersburg, Russia, early Thursday, May 11, 2017. North winds brought snow to the east of the St. Petersburg region. (Photo by Dmitri Lovetsky/AP Photo)