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)

Natalie Portman and Tessa Thompson speak at the Marvel Studios Panel during 2019 Comic-Con International at San Diego Convention Center on July 20, 2019 in San Diego, California. (Photo by Kevin Winter/Getty Images)

Festival goers wear Australia day attire at Big Day Out 2012 at the Sydney Showground on January 26, 2012 in Sydney, Australia. (Photo by Mark Metcalfe/Getty Images)

Stefan Libon proposes to girlfirend Jess McGarrity in front of a banner message “Marry Me Jess?” on the deck of the Queen Mary 2, at Mrs Macquarie's Chair on Valentines day, February 14, 2012 in Sydney, Australia. (Photo by James Morgan/Carnival Australia via Getty Images)

An African migrant stranded in Costa Rica bathes at a makeshift camp at the border between Costa Rica and Nicaragua, in Penas Blancas, Costa Rica, July 14, 2016. Thousands of African migrants are currently stranded in Costa Rica after they were denied entry into neighbouring Nicaragua, which they have been using as a crossing point in order to reach the United States. According to the Costa Rican Red Cross, most of these people from Africa, Asia and Haiti. It has been reported that many of them paid smugglers to reach Central America. However, when they arrived at the border gates, they were denied entry and were left with nowhere to go. (Photo by Juan Carlos Ulate/Reuters)

Spectators pass through security screening ahead of the New Year's Eve celebration in Times Square in New York, on Sunday, December 31, 2017. (Photo by Peter Morgan/AP Photo)

Photographer Lori Nix hand-crafted dioramas are fictional scenes of a post-apocalyptic world in which plants and decay reclaim both grand and mundane structures. Each structure is built in Nix's living room and can take over half a year to build. Photo: Library, 2007. Photographer Lori Nix says this is the most popular image she has made for this project that she calls “The City”. (Photo by Lori Nix)

Tank crew standing in front of M-4 tank, Ft. Knox, Kentucky, June, 1942. (Alfred Palmer/OWI/LOC)