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)

Students from Cambridge University in England make their way home along Trinity Lane after celebrating the end of the academic year at a May Ball in Trinity College on June 18, 2019. (Photo by Joe Giddens/PA Images via Getty Images)

Participants of the “Lady in red“ procession, marking the day of the Summer Solstice, walk on a promenade of the Yenisei River in Krasnoyarsk, Russia on June 22, 2019. (Photo by Ilya Naymushin/Reuters)

A man walks on a slackline during the 7th edition of the European “Marmotte Highline Project” (MHP) festival in Lans-en-Vercors, near Grenoble, eastern France, on July 4, 2019. The event, during which the participants will be able to evolve on the various highlines located in the Regional Natural Park of Vercors, takes place until July 7, 2019. (Photo by Philippe Desmazes/AFP Photo)

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)

Wasibi, (r), yawns at backstage before the Algonquin Hotel’s Annual Cat Fashion Show in the Manhattan borough of New York City, New York, U.S., August 1, 2019. The theme of this year's show was “It's a Small World”. The cats represented countries including Ireland, Spain, the United Kingdom and the United States. (Photo by Jeenah Moon/Reuters)

Freya Smith aged three, leads one of Erth’s giant dinosaur puppets across the road on August 6, 2019 in Edinburgh, Scotland. Australian theatre company Erth presents their best-selling show Dinosaur’s Zoo as part of Underbelly’s Fringe programme. Featuring giant dinosaur puppets which walk, roar and blink like the real thing, Dinosaur’s Zoo is a perfect example of edutainment for children of all ages, taking place at the McEwan Hall every day of the Fringe at 11am. (Photo by Jeff J. Mitchell/Getty Images)

A B-52 bomber is watched by a group of curious onlookers as it lands at RAF Fairford, February 21, 1999. The B52s arrived from the U.S.A. as part of NATO's preparations for air attacks on Yugoslavia as tensions in the Kosovo crisis increased. (Photo by Dylan Martinez/Reuters)