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 dancer performs before a judging panel during the World Irish Dancing Championships in Dublin, Ireland on April 11, 2017. The April 9-16 event draws some 5,000 performers from Ireland and overseas, including from both the Irish diaspora in places like the United States and Australia as well as less likely countries such as Russia and Japan. (Photo by Clodagh Kilcoyne/Reuters)

Fans dressed in Spider-Man outfits gather at a promotional event for the forthcoming “Spider-Man: Homecoming” movie at the Art Science Museum in Singapore on June 7, 2017. The movie will be shown in cinemas around the world from July 5 onwards. (Photo by Toh Ting Wei/AFP Photo)

Canada's Marissa Papaconstantinou makes her way to the finish line after falling in the Women's 200m T44 Final during day ten of the IPC World ParaAthletics Championships 2017 at London Stadium on July 23, 2017 in London, England. (Photo by Peter Cziborra/Reuters)

This handout photograph taken and released by Limex Images for Red Bull on February 6, 2019 shows France's Nicolas Ivanoff flying over the skyline prior to the first stage of the Red Bull Air Race World Championship in Abu Dhabi, United Arab Emirates. (Photo by Predrag Vuckovic/AFP Photo)

Frida Karlsson takes 2nd place, Therese Johaug of Norway takes 1st place, Ingvild Flugstad Oestberg of Norway takes 3rd place during the FIS Nordic World Ski Championships Women's Cross Country Classic on February 26, 2019 in Seefeld, Austria. (Photo by Lisi Niesner/Reuters)

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)

Participants of the the goth festival “Wave Gothic Festival” walk in fancy costumes along the festival area in Leipzig, central Germany, Saturday, May 23, 2015. About 20,000 members of the scene are expected to attend one of the world's largest gothic and “dark” culture festivals until May 25, 2015. (Photo by Jens Meyer/AP Photo)